The heroes encounter a situation where the villain is an imposter of one of their own and/or a sympathetic character. The heroes must find out about the imposter and expose him/her to stop him.
Typically, the situation will involve the impostor and the real person standing side-by-side, both claiming to be the real one, and the other heroes must use their wits to identify who is who, usually by finding a quality that the real person doesn't have (in comedic examples, a positive quality). Or the real person may ask that their friend shoot both of them just to guarantee they get the imposter (and it's almost inevitable if the person has a super-healing power), which usually results in the heroes shooting the other one, because the impostor wouldn't be noble enough to suggest making the Heroic Sacrifice. This is often parodied these days where the imposter suggests it, knowing they'll assume the noble act to mark them out as the original — and sometimes double-parodied when this ends up outing him as the fake since the real person wouldn't be so noble.
In videogames, often the two clones will be desperately attempting to beat the crap out of each other while the player struggles to figure out a means of telling them apart, often with one tackling the other, punching him a few times until the pair rolls over and the person on top switches, ad nauseum (which conveniently allows a very short looping animation to represent the whole fight).
Speculative Fiction variants can get really confusing once you add in the possibilities of magic and/or Applied Phlebotinum: For example, the victim of a Body Swap must find a way to convince their friends that the person who looks like them really isn't, while the impostor is trying to convince the same friends that the victim is trying to initiate a body swap instead of undo one.
A common subversion is for the protagonist to either shoot in a blind guess that turns out correct, or simply shoot both and see who gets madder. In both situations, the victim of the imposter is likely to be incensed that the hero couldn't tell him from his doppelganger.
Another subversion is for neither of them to be an imposter but in fact both are two parts of the same person split with Applied Phlebotinum; see Evil Twin and Starfish Character. If they are two (visibly) different people but both claiming to be the 'real' whatever-position-would-be-relevant (captain, mother, president, owner, etc), see Judgment of Solomon.
A more comical subversion is when the imposter looks nothing like the person they are imitating, but people still act like they are identical, even if they haven't bothered with a Paper-Thin Disguise. The less the two look alike in this situation the more likely it is that no one can spot the difference.
May be caused by a Shell Game with people or when a Kill and Replace situation has occurred.
See also: Evil Twin, Bluff The Impostor, Ten Little Murder Victims.
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Anime & Manga
In Naruto, two different characters use ninjutsu to disguise themselves as Naruto himself in the same episode, but one is identified as an imposter because he was left-handed (his throwing knife holder was on the wrong side), and the other reveals himself as a fake by repeating a long, complicated Trust Password that the real Naruto would never be able to remember.
The existence of a transformation technique in the series lead to several more instances of Spot the Imposter, both for mindgames in fights and for comedic filler.
In one episode of Ghost in the Shell: Stand Alone Complex, a woman who has obsessively studied Pazu has a duplicate of his cyborg body made and her brain put into it; the two then fight and one ends up dead; the incident is not brought up for the rest of the series, implying that either the real Pazu won, or the fake was so good that she simply took his place.
Judicious use of the pause button, combined with paying very careful attention to details such as wounds inflicted at the start of the fight (when viewers still know which Pazu is the real one), make it clear that the real Pazu won the fight. However, the show lets you figure it out for yourself instead of spelling it out for you.
In a Mazinger Z episode, Big Bad Dr. Hell fabricates a robot seems just like The Professor Yumi. However it was quickly identified by Sayaka when she noticed the imposter's nails were black. Dr. Hell used that strategy more times, creating a robot looked right like a secondary character and going as far as building a robot resembled The Hero Kouji. They also were spotted sooner or later.
That plot was partially reused in a Mazinkaiser episode. The Dragon Baron Ashura kidnapped Prof. Yumi and disguised himself like him. Sayaka suspected him nearly right away, though, because he tossed a birthday gift into a trash bin, and she told others her father would never do that.
Something similar occurs in the climax of Read or Die's OVA. By paying attention, it's possible to note that the "real" Miss Deep is the winner before it's revealed as part of the plot.
In an episode of YuYu Hakusho, Yuusuke is told that one of his friends is an imposter, and he must identify the imposter by punching them. He correctly chooses Kuwabara, but then reveals that he'd simply chosen the suspect who'd be best able to cope with being punched in the face. And the one who'd most likely fall into the trap allowing an impostor to appear in the first place.
Again the split was used in Ah! My Goddess, when Urd was split into her Demon and Goddess parts. Made easier when it turns out that Skuld had secretly put a mark on the goddess Urd.
In an episode of Rockman.EXE Beast, an evil alternate version of Numberman shows up infiltrating the core group of friends. When the normal universe's Numberman arrives, obligatory hilarity ensues as the two attack each other identically, to such an extent that their Dice Bombs roll the same number. The problem is solved when it's realized that if they attack both Numbermen, the fake one would be forced to shapeshift, and then all present protagonists can hit him at once.
However the evil Numberman has a different color scheme and is clearly wearing a symbol of the Gregia faction instead of Numberman's usual insignia so there's no reason they shouldn't have known he was fake the instant they saw him beyond the fact that the cast is terminally stupid.
In Futari wa Pretty Cure, a villain impersonates Honoka at one point. When Nagisa has to pick the correct Honoka, all she can come up with is to ask them what her (Nagisa's) favorite food was, which is a question that has some problems. The Honokas proceeded to switch off saying accurate (and, for a time, increasingly negative) things about Nagisa, until the real one finally remembered that only she could read the diary in which Nagisa wrote that "my socks are a little smelly," a quotation that confused the fake. (The episode featured a Red Herring in the form of a perfume Honoka had made earlier in science club — as both Honokas pointed out, having or wearing the perfume proved nothing, because the fake could have just stolen the bottle.)
Subverted in Fullmetal Alchemist when Mustang and Hawkeye are confronting Envy. Envy pretends to be Mustang in front of Hawkeye. However, after a brief exchange, she points her gun at him and says she knows he's not Mustang because he was calling her Lieutenant, while the real Mustang always calls her Riza when it's just the two of them. Envy expresses surprise that the two of them are that close, at which point she tells him she was lying, but he'd already given himself away.
Played straight in the 2003 anime version. In the two-part episode "The Other Brothers Elric", the Tringham brothers pose as Ed and Al. In a variation, the Tringhams don't even resemble the Elrics (save for hair color), but no-one in the town knew what the Elrics look like; they just know their reputation.
When Lust poses as Ross, Hughes knows it isn't her when he notices she didn't have a mole under her left eye.
In Zatch Bell!, one of the characters has the ability to change into other characters... badly. He often has a way too big nose or forehead, but everyone but the one he transformed into is oblivious to the differences.
One Pokémon episode is titled "Will the Real Oak Stand Up?", with James from Team Rocket being the impostor.
In the Alabasta arc in One Piece, Mr 2: Bon Kurei assumes the form of Usopp to fool Vivi. He fails because the crew was on guard, having met him before and had prepared a special sign to reveal their true identities.
In Sumomomo Momomo, Iroha uses her clan's ability to turn into Momoko, voice and all. She gets touchy feely with Koushi when in the presence of Sanae so Sanae will not interfere with Momoko and Koushi's future marriage. The real Momoko shows up and Koushi figures the right one because Iroha is grabbing onto his arm which was just injured the previous episode and the real Momoko would know about that since she treated the injury. The real Momoko thinks the fake one is an assassin and quickly blasts her away. Iroha shows up about 5 seconds later without the disguise, but very bruised. Nobody ever figures it out or asks about it again.
Subverted in a filler arc of Bleach where it turns out that both of them are evil clones.
A Fantastic Four and Spider Man story resolved the situation of the Chameleon impersonating Spider-Man in a distinctive manner. The Thing, holding one in each hand, throws both several meters into the air. The Chameleon reveals himself by screaming in terror and the Human Torch flies up to catch him while the real Spider-Man, who is used to this kind of situation, calmly shoots a webline to swing to safety.
This happens a lot when the Chameleon is involved. (He is a Master of Disguise, after all.) One time after he discovered the hero's identity (and the fact that he was Kraven the Hunter's half-brother, something which gave him his confidence back) he impersonated Spider-Man and tried to fool Mary Jane. She realized it wasn't Peter when he kissed her; as she said herself, "It made my skin crawl." (This led to what was probably the most humiliating situation in the villain's career. Spider-Man didn't need to do anything; she beat him senseless with a baseball bat.)
Gold Digger plays with this trope. Shapeshifter Madrid arranges to disguise herself as the Heroine, Gina Diggers. She then arranges for the real Gina to act out of character in several suspicious ways (an out-of-date outfit, etc.), while trapping herself for Gina's companions to find. As a finishing touch, during the confrontation with Gina, she uses the "shoot us both lock us both up" subversion to win the trust of the others.
In the Amazons Attack DCU Crisis Crossover, two Sgt. Steels have been discovered, and the government agency he runs need to figure out which is which. Finally, after a long interrogation, one of the Sgt. Steels points out in exasperation that he has a metal arm, which doesn't bleed — and then swiftly stabs the other Steel in the 'metal' arm with a pencil, showing the second Steel to be a shapeshifter when he shouts out in pain. This is subverted, however, in that the first 'Sgt. Steel' is ALSO an imposter, one who has taken on Steel's form to discover where the real one is; he just happens to be an imposter who's quicker at thinking on his feet than the first one.
Somewhat complicated version in a Nick Fury comic: Fury is battling Baron von Strucker, the head of HYDRA in his soon to be Collapsing Lair, but he has to disguise himself as Strucker using a convenient masking device in order to commandeer a plane out. However, he also sets up Strucker's death by first disguising Strucker as Fury and then putting a second mask on Strucker so he's wearing his own face on top of Fury's. Result? HYDRA troops burst in and see two Struckers fighting. Fury-disguised-as-Strucker convinces the HYDRA troops he's the real Strucker by yelling louder and ordering troops to remove Strucker-as-Fury-as-Strucker's mask, so the troops comply. Strucker panics and dives into the alpha particle generator, dying. Fury-as-Strucker jets off.
Archie Comics Sonic The Hedgehog had an extended version in which the changes of Antoine were revealed to be him having been replaced with his Mirror Universe counterpart, who had promptly set to political maneuvering and nearly arranged to become king of Mobius before Sonic finally Spotted The Impostor. (Antoine, meanwhile, had been dumped in his counterpart's place and had been doing his best to play evil to keep anyone from catching on.)
An early Batman serial strip had Batman disguise himself as a mob boss in order to free Robin - unfortunately, the real one came back earlier than expected, and the boss' henchmen had recently decided to get rid of their Bad Boss.
An example occurs in the Batgirl Adventures Annual #1 comic where Batgirl is helping Harley Quinn track down Poison Ivy, who has been abducted. The villain manages to copycat Harley completely, from attire to voice, and Batgirl contemplates just beating up both of them. She finally figures out the real one when Harley uses her insult nickname for B(r)atgirl.
In an issue of a short-lived mid-70's comic named after and starring The Joker, he decided to hold a trained cat belonging to a big-name movie comedian for ransom. The comedian disguised himself as the Joker in an attempt to retrieve the animal, and when the Joker's minions were confused, the Joker himself suggested that they let the cat go and whoever it went to would not be the Joker. When it went to the Joker instead, and he protested (after being arrested) that a cat couldn't be trained to go to someone other than its master, the comedian said something like "No cat except my million-dollar-kitty!"
A heartbreaking one during Marvel's Secret Invasion (the main mini-series): In Issue #8, Back from the DeadMockingbird is confronted by her ex-husband, Hawkeye (who, at the time, was presumed dead and operating as masked hero Ronin). To prove she wasn't a Skrull, Hawkeye states a certain date, and asks Mockingbird what it meant. Turns out, Mockinbird had a miscarriage on that date and only she and Hawkeye ever knew she was even pregnant. This also served tell Mockingbird who was under the Ronin mask.
An early issue of the IDW reboot has him try to get into the team disguised as Snake-Eyes, only to get clocked by Duke. Duke didn't know he wasn't really Snake-Eyes, but was simply suspicious: If it had really been Snake-Eyes, he'd have easily blocked or avoided the punch.
Piker: Captain, what is my favourite trombone jazz number? Pinchhard: How the hell should I know?! Pinchhard 2: Beats me. Piker: I played it for you on your birthday! You loved it! Pinchhard: Oh please! I slept through the whole thing! Pinchhard 2: Here's a combadge, Number One. Call someone who cares!
Toy Story 2. When the toys realize they have two Buzz Lightyears, the real Buzz cleverly reveals his double's true identity by pressing the button that opens the double's bubble helmet. The double falls to the ground gasping for air, the same way the real Buzz did in the first movie. He clinches the identification by showing the toys the name "Andy" on the sole of his boot.
Films — Live-Action
X-Men has Mystique pulling that trick too. However, Wolverine can smell the difference between Mystique and Storm, so he doesn't fall for it. Later, Cyclops asks Wolverine to confirm he is the real thing:
Wolverine: You're a dick. Cyclops: Okay.
In X2: X-Men United, the trope is averted when Stryker is able to recognize his "handiwork" on sight — it's enough to fool the soldiers, but somehow the Colonel can tell with one close-up look. Which actually isn't that surprising. Parents can tell the difference between identical twins even when others can't.
Done in the movie The Island, with a twist: the impostor was the protagonist.
Darkman did the same thing. His cover was blown, however, when his skin started melting.
Clumsily handled in Futureworld, the deservedly forgotten sequel to Westworld. The first movie, competently scripted and directed by Michael Crichton, features an average man (Richard Benjamin) being stalked by a gunslinger robot (Yul Brynner) through a robot-filled theme park. The robots are just enough "off" from real humans to get on your nerves. The sequel dumps all the subtlety and make the robots "perfect" imitations of humans, so it can save on acting and direction cost and steal the plot of Invasion of the Body-Snatchers. Late in the film, the two humans escape into the theme park and the boss robot, instead of ordering his trustworthy minions after them, sends out their own robot duplicates! This bit of Plot Idiocy telegraphs the movie's Spot the Impostor ending so ludicrously you want to shout "Oh, come onnnn!" at the TV set.
In keeping with the love that Star Trek: The Original Series had for this trope, it gets used again in Star Trek VI: The Undiscovered Country, at the climax of Kirk and McCoy's escape from Rura Penthe. A shapeshifter impersonates Captain Kirk, but it is the real Kirk the Klingons want to shoot. A "Shoot him he's the one" argument ensued, with an element of Fridge Logic: the shapeshifter should have tried to save herself by switching back to her normal form. She still would have been shot, but there's no explanation for why she doesn't.
Face Off features a face-switch between the protagonist and antagonist, which both use to fool those close to their enemy. Hero Sean Archer convinces his wife of his true identity with the story of their first kiss.
Terminator 2: Judgment Day: Page quote of Bluff The Impostor... and the closing fight scene where the shapeshifting Terminator has impersonated Sarah Connor, and John must work out which is the real one (which wants him alive) and the bad one (who wants him very very dead). He picks the good one.
In the extended version, they reveal how John was able to make the right choice: due to the freezing/smashing/recombining the T-1000 went through, its morphing is damaged and it ends up copying the appearance of materials it is in contact with. John looked down and noted that the fake Sarah had a corrugated metal pattern on its legs. In another version, the heat rising through the grating below has melted her legs in the pattern of the grate.
Red Dawn 1984. One of the American guerilla fighters finds an U.S. Air Force officer who's been downed behind Soviet lines and asks "What's the capital of Texas?" When he replies "Austin" she replies that it's Houston and accuses him of being a Dirty Communist. Tanner replies that she's been watching too many movies. (Ironically Tanner gives the right answer, probably a Shout Out to the incident with General Bradley at the Battle of the Bulge.)
Happens twice in The Assignment (1997) about a US naval officer called on to impersonate Carlos the Jackal. On the first occasion a terrorist who knows the real Carlos accidentally runs into the protagonist at London airport. Unfortunately the protagonist tries to bluff his way out by pretending to be Carlos, and when the terrorist responds, "I need to get a newspaper" realises too late that it's a password to which he doesn't know the countersign. Later his CIA handler mentions a similar incident where he was forced to kill a man who didn't respond with the correct countersign, and later uses this story to tell the difference between the protagonist and the real Carlos.
The blood test scene in The Thing (1982) qualifies. In fact, given the creature's ability to disguise itself, this is a constant theme of the entire film.
If the blood test counts, so does any film of TV show that homages the blood test scene, such as The Faculty, with the drug test.
Police Academy 6: City Under Siege ends with the Big Bad posing as Commissioner Henry Hurst. To see who the fake one is, the police officers use the Pinocchio-test (they perform a nose pull on both commissioners, revealing the fake one is wearing a rubber mask).
The Alex Rider book Point Blanc ends with a genuinely disturbing sequence in which Alex and his Doppelgänger go mano a mano. The prose deliberately acts like a distant camera, leaving the reader to play Spot The Doppleganger and find it impossible, even when one dies. In the next book, everything's all fine and dandy with Alex being the survivor, but...
In the Thursday Next book First Among Sequels, Thursday fights off a fictional duplicate of herself. As a twist the writing changes so that we see things from the duplicate's point of view, despite being in the first person narrative, and without warning. It can take a reader a while to notice the switch.
Meg must perform this task in Madeleine L'Engle's A Wind In The Door, distinguishing her detested principal Mr. Jenkins from two imposters created by echthroi (which are essentially that universe's equivalent of demons). The imposters try too hard to be Mr. Jenkins and so fail, while the real Mr. Jenkins simply acts like himself (bored, confused, and wondering when this little game will be over).
Averted in Brothers in Arms by Lois McMaster Bujold. Miles is kidnapped, replaced by his clone, and interrogated; at one point in the interrogation, he and the clone are briefly alone together. He considers trying to trick his captors into thinking that he's the clone, but rejects the idea when he realizes that he hasn't shaved since he was taken and is currently sporting several days more stubble than the clone is.
The Iron Man novel Operation: A.I.M. had several Avengers battling shapeshifting "adaptoids"; Iron Man tricks the War Machine impostor into giving himself away by pretending his Powered Armor had a chameleon circuit. When the android uses this as an excuse to shapeshift in front of the others, his cover is blown. Later, the heroes must fight their own doppelgangers and resort to uncharacteristic tactics to win since the fakes know all their usual moves.
Subverted in Maximum Ride. After the situation is set up, the imposter is instantly identified by Angel, who points out "I can read minds, you moron."
In the David trilogy of Animorphs David morphs Jake's cousin, who is badly injured after being hit by a car and expected to die in surgery, and takes his place. Jake figures it out almost instantly, when "Saddler's" injuries are miraculously healed. Everyone else not in on The Masquerade fails miserably.
David also impersonates Marco. When a Marco shows up, he's tested by the others: they make comments to him and see if he responds sarcastically.
It also gets some time in the Harry Potter books because of the Polyjuice Potion: at the start of Order of the Phoenix, where Lupin asks Harry what his Patronus is, several uses after the Seven Potters escape, and a number for trying to detect Death Eater impersonators.
Stay Out of the Basement involves a plant clone. After the clone is revealed to be a plant being, the plot seems to wrap up... until a flower in the front garden attempts to convince the protagonist that it is the real one.
In I Am Your Evil Twin, the protagonist uses his food allergy to prove that he's not the evil clone. Unfortunately, the clone swaps the target food for a version the protagonist is not allergic to, and then vomits on purpose, successfully fooling everyone into thinking he's the real thing.
Spoofed in The Culture novel Matter when Upper Class Twit Prince Ferbin is on the run and seeks help from a former tutor who, to verify his identity, asks him a series of questions from his studies. When he gets them all wrong, the tutor responds dryly that Ferbin is indeed his indifferent student from long ago.
Played with in the Phule's Company book Phule me Twice, in which the identical looking robot is never recognised, despite its limited programmed responses, which are all casino based. Even the Company smart folk don't get suspicious, when advised to try the slots or the all-night buffet. Instead, they fear sunstroke. After Phules return, they use the robot to drive the unwanted new CO over the edge, as he wouldn't know about its existence.
In Sewer Gas And Electric, Harry Gant is kidnapped and forced to compete against his robot duplicate in a video game, with his parents' lives as the stakes. His ex-wife breaks in to rescue the Gants, and the robot tries to trick her by voicing its "relief" at being "saved". She immediately shoots the robot, knowing that the real Harry is such a game addict that he wouldn't even notice her arrival.
God Emperor of Dune inverts this, where there's an army of impostors and the task is to spot the real one. Leto II's Fish speaker army is facing a legion of shapeshifting Face Dancers who are impersonating his general, Duncan Idaho. When he scans the crowd, he immediately spots a naked one among the copies.
In a Nancy Drew book, when referring to a man who had been hospitalized, another man says, "taken to hospital", instead of "taken to the hospital". The former is a British term. The man has been posing as an American to escape crimes he was charged for in England.
Only resolved through a Deus ex Machina in Journey to the West when two identical monkeys claim to be the monkey king, and they take their fight to continuously more powerful divine beings, all of whom (except the last) fail to identify the real one.
Subverted in A Farewell to Arms by Ernest Hemingway. The protagonist (an American volunteer in the Italian army) is arrested as a German infiltrator because he speaks Italian with an accent — as does the military policeman arresting him, as most Italians were more familiar with their local dialect.
Partially subverted in the first story in Isaac Asimov's The Union Club Mysteries, where a club member named Griswold commented that spies sent to infiltrate defense agencies in particular were recruited from among people who'd spent several years in the US and were thoroughly grounded in American trivia. He then told of how he'd unmasked one such spy whom he'd been interrogating for hours by playing a word-association game which finished with him saying "terror of flight" and the spy responding "gloom of the grave." This was a giveaway because both phrases are from the third stanza of "The Star-Spangled Banner," and "...no loyal, true-blue American knows the words of the third stanza of our glorious national anthem" making it much more likely that it was a spy who had studied too hard.
In one of the Clue mystery series stories, Professor Plum makes several clones of Miss Peacock, alike in every way, save that the clone-Peacocks always lie. They corner all of them, all acting like fuss-budgets, all claiming to be the real Peacock, and the reader is asked how to tell them apart. The answer is to ask any simple question with a non-subjective answer, like "What is 2+2?".
Implied to happen offscreen in The Dresden Files, and rather frequently, at that. Ghost Story, covers how Chicago has gotten a great deal Darker and Edgier since the events of the previous book, Changes, six months previous. In order to verify a character's identity, Karrin Murphy demands him to "Bleed for me." Not only does he do so without protest, he has a pin on him for such a purpose. Harry notes that, while there are a great many supernatural shapeshifters, none of them can create convincing blood. He also comments on how bad things must have gotten in the six months he's been dead for the test to have gotten so commonplace.
Galaxy of Fear: The Nightmare Machine has an elaborate and convincing simulation going on in which the protagonists' worst fears are realized. Zak realizes that his robot caretaker DV-9 is actually a sim because Deevee never reacted to the simulations before, among other things.
In Clones, the original and clone Hooles had fought a Shapeshifter Showdown; Tash had been watching, both because it was fascinating and because she didn't want to have to do this trope when they were done, but is interrupted and loses track. Soon she does have to spot which one is real... and comes up with something.
"There's no way to tell you apart," she said, raising the blaster. "I'll just have to shoot you both."
The clone jumps at her, so she blasts him. The real Hoole asks how she knew.
Tash shrugged. "Clones don't really understand feelings that well. The real Hoole would have known I couldn't shoot him, but the clone wouldn't know that. I knew he would believe my threat, and try to stop me."
Earlier in that book a clone of Zak wants Tash to enter a code, but she stops warily, thinking that he's supposed to know the code too, and why wouldn't he have done it himself? These clones have many of the same memories as their templates, but apparently in less detail.
In the story "The Brain Stealers of Mars", the thushol are Martian shapeshifting parasites who impersonate the heroes. They're also mind-readers, so that a duplicate has all the knowledge of the original, and the heroes can't find out who is the real one by questioning each other.
In the early story Counterfeit, an alien imposter who has infiltrated the crew is detected in a medical check because the imitation isn't quite perfect; amongst other faults, the alien has a blood-glucose level of zero — a real human would be dead.
In the episode "Whom Gods Destroy", Spock sees Kirk standing right next to an insane shapeshifter who is posing as Kirk. Spock identifies the imposter getting into a fight and noticing that one Kirk orders them both shot to prevent the imposter from escaping. Knowing that the imposter would never give that kind of command, Spock stuns the other one. This may be the origin of the "shoot us both" gambit, which itself is so well-known that today it's more likely that the evil one will use it, expecting the decider to shoot the other one (several such examples are below).
Spock knew that the shapeshifter in question couldn't hold another identity for more than a few minutes. He says so, and explains that all he has to do is wait. That's when the "Shoot him! No, shoot us both" dialogue occurs.
Leonard Nimoy hated this episode, noting that as The Smart Guy Spock should have been able to easily and quickly create the kind of highly personal trick questions only his best friend, Kirk, should be able to answer properly to identify himself. According to Spock, he did not make his choice based on the order to shoot them both, but rather based on which one was winning: Kirk was recovering from an illness and thus was at a disadvantage against the healthier duplicate.
A variation of the above gambit happened in the episode "What Are Little Girls Made Of?": The villain creates an android clone of Kirk with a copied mind, but Kirk fills his mind with anti-Vulcan racist thoughts when the copying procedure is happening to contaminate the android's mind. This causes the android to insult Spock, which causes Spock to realize that the android is an impostor.
One of the worst episodes of the series, "Turnabout Intruder", used the body-swap variant.
The first episode ("The Man Trap") featured a shapeshifting creature that drained the salt from people. It shapeshifted several times before settling on shifting into McCoy's form. It could be spotted by its tendency to curve its index finger and nibble slightly on the arc of the finger.
Actually, that gesture was meant to be an outward expression of its craving for salt. See also the way alcoholics were often presented on film in that era.
An asexual instance of Freud Was Right? Id and Superego split from the Ego? Oh hey, Incredibly Lame Pun.
And again in "Mirror, Mirror", when Kirk, McCoy, Uhura, and Scotty are transported to the mirror universe and replaced by their doubles. Spock casually mentions at the end how easy it was to spot them, saying, "It's far easier for civilized people such as yourselves to act like barbarians, than it is for barbarians to act like civilized people."
Spock: They were brutal, unprincipled, uncivilized, treacherous; in every way splendid examples of Homo sapiens, the very flower of humanity. Kirk: I'm not sure, but I think we were just insulted... McCoy: Oh, I'm sure of it.
Star Trek: Deep Space Nine did an Arc about shapeshifting imposters. However, the first episode in the arc ("The Adversary") was actually a subversion. A Founder traps the crew inside a ship it controls, then tries to pick off the crew one by one. The climax of the episode is O'Brien seeing two Odos, who demand he pick the real one. O'Brien shrugs, says "I've got better things to do than play 'choose the changeling'." and begins reprogramming the ship's computer — which forces the imposter to reveal himself by attacking.
One episode had Sisko and a Jem'Hadar grappling, while an ally stood nearby with a phaser. The visibility was so poor he could only make out silhouettes. He found a way to shoot the right one:
SISKO: You've got sharp eyes. EDDINGTON: Not really. I just waited to see which of you was knocked down first, and then I shot the one still standing.
In a weak first-season episode of Star Trek: The Next Generation Lore incapacitates Data and proceeds to impersonate him. Handled in a terrible, confusing way: Wesley quickly notices the impersonation, but Picard is inexplicably unwilling to believe him, telling Wesley to "shut up."
In The A-Team, B.A. faces a sympathetic character who is standing right next to his imposter, who resembles him in every way, except that they each have different colored lapel flowers. He claims that he can use those flowers to identify the imposter, a bluff based on the (successful) gamble that the imposter would bolt in a panic, thus revealing himself.
Red Dwarf, "Psirens": The fake Lister is identified because he can actually play the guitar.
The Bionic Woman and Gemini Man both shared a Recycled Script in which each show's hero must cope with an identical imposter. The Bionic Woman proves her identity while standing side by side with the imposter by calling attention to herself and then jumping several metres up, a move the imposter cannot duplicate, thus exposing her. The hero of Gemini Man simply turns on his invisibility.
There was a second episode of The Bionic Woman involving Jaime's imposter, who was now taking pills to duplicate her bionic strength. When Jaime was cornered by Oscar and the authorities, she proved she was the real one by telling Oscar to ask her a question without using the bullhorn he had, which only she could have heard because of her bionic hearing.
Stargate SG-1 has the episode "Holiday", where Daniel is tricked into switching bodies with an aging scientist who wants to have a little fun before he dies. Though the two are different characters, they are both played by the same actor.
In that episode, Daniel (in Machello's body) is questioned to prove whether he's really Daniel or not:
Jack: Okay, what color dress did your sister wear when I went out with her last week? Daniel: I don't have a sister, and if I did, I'd never let her date you anyways.
Jack is once transformed into his teenaged self (sort of, it is a clone, but we don't know that until the end of the episode) and starts revealing classified information about the rest of the team to prove who he is.
There is another one where Daniel is invisible and communicating through his grandfather, who is the only one who could see him. Spot the Imposter without the imposter.
Daniel: Repeat what I say. I'm standing right next to you. Nick: He's standing right next to me. Jack: Lost a little weight, have you? Daniel: Jack, don't be an ass. Nick: Jack, don't be an ass. Jack:(looking towards Daniel in surprise) Daniel?
Daniel was just muttering the second phrase to himself, not expecting Nick to repeat it.
Angel's season 3 episode "Carpe Noctem" featured an old man called Marcus who switched bodies with Angel.
Marcus-in-Angel's body: "It's him — he's the one who's been casting that spell." Cordy: "You're Angel? With that cologne? I don't think so."
Seven Days had a recurring imposter character, Galina Komanov, who looked exactly like the main character Dr. Olga Vukovich. She appeared in the first-season episode "There's Something About Olga" and the second-season episode "Two Weddings and a Funeral".
Noticeably subverted in the 1995 Are You Afraid Of The Dark episode "The Tale of the Chameleons", where it appears that the imposter actually manages to survive while the impostee is killed. Also notable for the fact that the imposter and impostee were played by different people (namely the Mowry twins).
In an episode of Get Smart, Max is unable to prove himself legitimate, as the imposter who replaces him is well-informed. Why no-one finds it suspicious that normally lame-brained Max would be so up on top-secret information is a mystery.
Get Smart must have the highest ratio of evil imposter plots, Max, 99, and the Chief all being impersonated at least twice in its 5-year run.
The fifth season finale of LOST, "The Incident", provides an interesting variation on this. At the end of season 4, it was revealed that John Locke was dead. However, in the fifth season, after the Oceanic 6 return to the island via a plane that was carrying John's coffin, John is revealed to be very much alive. This is lampshaded by Ben, who is just as confused as the viewers, saying that the island has never brought someone back to life before. In the season finale, it is revealed that this John Locke is actually an imposter (Jacob's as-yet unnamed rival, who was looking for a "loophole" to kill Jacob). The body of the real Locke was still in the cargo hold of the plane, and it is not yet known how this man could have turned himself into an exact duplicate of Locke.
As it turns out, the imposter is Jacob's twin brother, also known as The Man in Black, also known as the Smoke Monster that had been antagonizing our heroes since Season 1. The Man in Black has the ability to take on the form of any person who had died in order to manipulate their loved ones into doing his bidding; after Jacob is murdered, he is somehow "locked" in the form of Locke for the rest of the show. However, the other characters eventually discover his true identity, aided by the fact that he slips into his smoke form from time to time in order to slaughter anyone who stands in his way.
The split personality type is used in Buffy the Vampire Slayer, when Xander is split into his "Cool" and "Lame" sides by a magical artifact. We follow the lame Xander around and discover suddenly that they're the same person. Surprisingly, the dual body effect was done so well because the actor Nicholas Brendon is actually an identical twin whose brother guested on the episode, although the twin was mainly used as a body double — Nicholas Brendon played all of the scenes in which one twin was alone, and whichever role was larger in the scenes where they were together.
The episode where Giles is turned into a demon, Buffy is able to tell it's him when she looks into his eyes "because no one else can look quite that annoyed with me."
Something similar happens when, after being attacked by zombies, Cordelia suddenly runs into Giles.
Cordelia: How do we know it's really you and not Zombie!Giles? Giles: Cordelia, do stop being tiresome. Cordelia: It's him.
Faith and Buffy switch bodies. When told by Buffy (in Faith's body) to ask her a question only she would know, Giles asks, "Who's President?" "We're checking for Buffy here, not a concussion," Buffy replies. Giles eventually gets it. Also, Joyce gets suspicious and Tara knows right away (because Buffy has the wrong aura). No one else clues in, however.
When Spike, unknown to the rest of the protagonists, has a robot double made of Buffy and has it programmed to sound and act like the real Buffy (with some "upgrades", like being nice to Spike instead of beating him up), the others fall for it when they first encounter the double, which gets Buffy mad, "You couldn't tell me from a robot?" (Given the poor quality of the impersonation, she had a point).
Well, Joyce had just died, so they expected her to act differently. And they did think she was insane!
In "Doppelgangland", Buffy and Xander are traumatized when they meet an alternate universe counterpart of Willow who's a vampire. When the real Willow appears in the library, Xander reacts by brandishing a crucifix, but when it doesn't cause Willow to recoil in pain, Buffy realizes that Willow hasn't been turned after all. Angel was also thrown by Vampire Willow's appearance:
Angel: Buffy, I... I just... Something's happened that...Willow's dead. (distractedly) Hey, Willow...Wait a second...
Xander: We're right there with you, big guy.
Spoofed in "Gone" when the Trio accidentally turn Buffy invisible. Andrew comments that Buffy could be anywhere, even right there among them. "For all we know, she could even be one of us!" The Trio look at each other in alarm until they realise how ridiculous that sounds.
The same plot is used in The Commish with twin brothers confessing to the same crime (a case of Artistic License - Law as both could be arrested for conspiracy). The real murderer is uncovered because he's dyslexic, and when telling his brother about the crime he passed on his incorrect reading of a sign.
Subverted in the final episode of the second Blackadder series: Blackadder tells a would-be assassin that Queen Elizabeth's nursemaid will attend her costume party dressed as a cow. After he escapes, Edmund hurries to the party and stabs the person in the cow costume. The others think he's killed Nursie... until she rushes in, wearing four costume udders, at which point Edmund points out: "Ludwig was a master of disguise, whereas Nursie is a sad insane old woman with an udder fixation."
Painkiller Jane has a variant of the "shoot them both" scheme where it is viable because Jane has regeneration powers. The imposter has regeneration powers too, but he does the Shape Shifter Swan Song and its all a plan bordering on roulette anyways, so it works out well for the team.
Scarecrow and Mrs. King Has an episode with an impostor Amanda Both Amandas are hanging off a building's roof and Lee is trying to figure out which one to save. He grabs the Amanda that says "Oh my goodness" because only Amanda would say that in this situation
Gilligan's Island had several impostor episodes. One featured an impostor Gilligan, who was actually a Russian spy. Gilligan discovers him and the two do a "mirror scene" in a doorway until Gilligan pretends to sneeze, but the imposter actually does sneeze. A second featured an imposter Ginger, who couldn't see well without her glasses, which gave her away. The third featured an imposter Mr. Howell, who first turned up on the mainland claiming he was the only survivor of the castaways, then wound up on the island after falling off his yacht.
Partially subverted amusingly in LazyTown when Robbie Rotten impersonates Sportacus. The other characters can't tell the difference despite Robbie being, among other things, 4-5 inches taller and a lot less muscular. Stephanie suggests a race to tell them who is the real Sportacus, and Robbie wins because Sportacus is forced to forfeit the race halfway through to rescue Bessie. Stephanie realises that the one who forfeited to rescue Bessie must be the real one, says so, and unmasks Robbie by pulling his false moustache off. The tag would be a very touching Aesop on friendship if it wasn't for the simple fact that Robbie and Sportacus look nothing alike, and theyshouldhave been able to tell them apart by looking at them.
Then there's the episode 'Double Trouble' where Robbie impersonates the mayor, and once again everyone falls for it, despite the fact that Robbie looks nothing like the mayor.
In the Law & Order episode "Brother's Keeper", the detectives found that their gangster murder suspect has a respectable look-alike younger brother who could have committed the crimenote One of the crimes. The original murder was of a drug dealer. The second was of a witness to the first crime. The respectable brother committed the first crime (the dealer sold a lethal dose of drugs to his daughter) and the gangster killed the witness to cover the other's tracks. The cops have a witness, but the gangster's lawyer forces the cops to include both brothers in the line-up. As it turns out, the witness is a nurse and could easily tell the brothers apart because the gangster is a heavy drinker with all the body wear from the habit.
In Sliders, Professor Arturo meets his alternate-universe-evil-self, and with the gang about to depart forever to the next Alternate Earth, the two engage in fisticuffs and one Arturo dives through the extra-dimensional portal, which then closes, stranding the other. We're never, ever told if the team got the real Arturo or the alternate one, but he dies anyway so it probably didn't matter.
It's hinted a few times in the following two episodes that they took the imposter instead (Arturo was an established football fan in previous episodes but here he wasn't) until the subplot was dropped. A later interview confirmed that they were supposed to have taken the imposter which would have been followed up had the show not gone off in the direction that it did. It was never said if it was only intended to be or if the Arturo that died was the imposter and the real one still lived.
The typical way to expose an imposter in Sentai shows is for the leader to have the team assume their standard Super Sentai Pose. The impostee will fall in line with the team while the imposter is lost.
This was the entire premise of Kamen Rider Blade's Hyper Battle Video, "Blade Versus Blade", in which one of the Trials becomes an evil copy of Kazuma (and by extension, an evil copy of Blade). Played for laughs all the way, his comrades' attempts at identifying the real Kazuma result in him getting shot in the ass and smacked on the head, despite the fact that the copy is wearing an extremely obvious red scarf. When resident Anti-Hero Hajime comes along, he easily identifies the real one by calling out Kazuma's name; the superhuman reflexes of the Trial cause it to react first, but as Kazuma later complains, "That kind of makes me look slow, doesn't it?"
This happens a lot in Kamen Rider Kabuto thanks to the fact that the main villain faction are shapeshifters who can copy a host's memories as well as their appearance. The most notable example comes when they imitate one of the main Kamen Riders so well that even the sentient Transformation Trinket is confused. His Mysterious Waif sidekick, however, knows exactly which one is the fake. The fake is right-handed, while the original is left-handed. This is never pointed out in the show, making it more of a bonus for eagle-eyed viewers.
Interestingly, The Famous Jett Jackson had one in The Movie finale where Jett (the actor) met Silverstone (the character he plays) for real as well as an evil shapeshifter, resulting in the shapeshifter changing to match the two already-identical good guys and complicating the crap out of the bit. To explain, Jett has to guess which one is Silverstone while Silverstone is guessing which one is Jett and anyone who walks in the room is guessing which one is Jett, which one is Silverstone, and which one isn't.
In Xena: Warrior Princess, Gabrielle is thought to be dead for a couple episodes, and when Xena thinks she's found her again, it turns out to be Gabby's identical (and evil) daughter Hope. Something of a subversion, as the actress was specifically instructed to play the role exactly as if she really was Gabrielle, as they thought that Hope should have been able to imitate her perfectly.
Alias had a technology, Project Helix, based around turning people into physical doubles of another. This technology was used no less than four or five times in the course of the show; most notably in the case of Allison Doren, who killed and replaced Sydney Bristow's best friend, Francie. Immediately upon getting a message warning her of the switch, Sydney casually offers her ice cream to "Francie" and goes off to find a weapon. Too late does Allison realize her mistake ("Francie doesn't like coffee ice cream."). Cue Cat Fight.
The Andromeda episode Double or Nothingness ends with a final test by fanatic gamblers where Dylan fights Dylan. But before they fight, real Dylan tries to seduce Romie and gets close to her. When the two Dylans separate, without saying anything Romie shoots the fake one, then says "I read your biosigns" while the real Dylan stammers to explain why he acted so weird.
Also used to a lesser extent in the episode "The Warmth of an Invisible Light", where Dylan, in an alternate reality where he's long dead, confronts Andromeda's AI. The AI tries to strangle his computer self (how that's even possible is questionable) since it assumes the only logical explanation is that he is a program made to look and act like Dylan, but while he's choking, he lists off information that only him and Andromeda would know.
Referenced in an episode of Monk, where Randy asks "Which is the real one?" after barging in on a pair of twin sisters, one of whom committed murder. They decide to arrest the one who wasn't being drowned.
McMillan and Wife has a double of Mac who appeared in two episodes. Enright is able to distinguish the two by the use of the phrase "peach cobbler" (his favorite dessert), although for some reason this keeps working even after the double has heard it.
In The X-Files two-part episode "Dreamland," Mulder and a Man In Black called Morris Fletcher have had their bodies switched. Mulder tries unsuccessfully to convince Scully of this by bringing up her full name, her badge number..."I have no idea what your badge number is!" and her recent taste in yoghurt. When Agent Scully is still skeptical he remarks dryly that at least she hasn't changed. Eventually Scully is convinced by the way "her" Mulder is a) repeatedly calling her Dana, b) sucking up to the Assistant Director, c) and coming on to her.
The Goodies had an episode called "The Baddies", where robot doubles of the lads were made. Tim eventually yells that they should try and unscrew the doubles' heads, to which the inventor of the doubles panicked, revealing them. (However, Rule of Funny set in during the following chase scene, where the boys routinely confused the fakes for the real ones, thus ambushing the wrong people, etc.)
Fringe season 2 ends with the team returning from the Alternate Universe, with Olivia's Evil Twin having switched places with her. This is known to the audience but not the other characters. In Season 3, the alternate Olivia has noticeably different mannerisms; she's not as cold, and seems more feminine. It's not immediately apparent whether this is merely a reminder to the audience or is noticeable to the characters. When it becomes evident, she explains it away by saying her experiences in the alternate universe changed her. She gets away with it to the extent of getting intimately involved with Olivia's Fringe-team partner, and is not caught until the real Olivia shows up. (at which point a Bluff The Impostor game ensues).
On the other side, some Applied Phlebotinum allows the original Olivia to remain completely undetected by those in the Alternate Universe who don't already know, despite some revealing mistakes.
Cleopatra 2525 has both the original and the imposter believing they are the true version.
Power Rangers has used this one a few times. The Primator (who had impersonation as his power) gave himself away while pretending to be Tommy by saying the rangers should give up (Tommy is a Determinator who on several occasions in the series, both before and after this moment, was willing to fight even without his powers, so giving up is so out-of-character for him it had to be the Primator). Also, while Primator disguised as Trini, Jason challenged both Yellow Rangers to fight him, saying he can easily recognize her fighting style. However, Trini proved her identity by refusing to fight, lest she harm a fellow Ranger, while the Primator leaped towards Jason to attack him with way too much enthusiasm for just testing Trini's identity.
In "Master Vile and the Metallic Armor," Tommy journeys into the Caves of Deception to retrieve the Zeo Crystal. Among the illusions he encounters are of the team and then just Catherine. They're brief, but pretty dead-on. Tommy sees through the illusions because of their willingness to surrender and professed belief that evil cannot be defeated.
"The Green Candle" two-parter gave us Cyclops, a monster that took the forms of Megazord, Dragonzord and Dragonzord in Fighting Mode. In Part 1, when Cyclops first appears in the form of Dragonzord, the other Rangers fear that Rita managed to turn Tommy evil again. (A quick scan leads to the truth, though.)
In an episode of Power Rangers in Space Astronema impersonates Ashley with the help of a monster. Then, while captured, Ashley tricks the monster into making her look like Astronema. Ashley arrives just as Astronema (while morphed) is about to shoot Andros, and reveals the truth by mentioning what Andros had gotten Ashley for her birthday (a necklace from his home planet, KO-35). Then, when Astronema de-morphs, she's back to looking like herself again, and the two start cat-fighting (seriously), at which point the other rangers arrive. Andros then figures out which Astronema is Ashley by shouting the team's Invocation, "Let's Rocket!" Ashley gets the morphing pose right, while Astronema is completely lost.
In an episode of the second season, Kimberly tells the real Billy apart from a copy made from a statue, by asking them to identify Billy's science project.
The Dukes of Hazzard: The episode "Too Many Roscoes." Not a comic episode where Rosco is somehow duplicated in some weird contraption and then wreaks havoc on Hazzard County. A double DOES wreak havoc, however ... via plastic surgery and a sinister plan (along with his cronies) to rob Hazzard Bank. The set-up: The real Rosco's patrol car is run into the lake, and the sheriff is promptly kidnapped by the bad guys, which includes Woody (James Best in a dual role), the bank robber who had plastic surgery specifically to prepare for this heist. Rosco — thought to be dead, then discovered alive and stumbling down the street — fools everyone in Hazzard (especially Boss Hogg and the Duke boys) by bungling simple facts, but the tip-off that "Rosco" wasn't who he said he was (remembering in exact detail the schedule arrival of an armored car and a $1 million shipment) goes unnoticed. Eventually, Woody exposes himself to the Duke boys and takes them to the same place where the real Rosco is being held captive. The Dukes and Rosco escape and team up to capture the bank robbers ... with the real Rosco punching out the phony.
Another episode featured a visit from Boss Hogg's identical twin brother, Abraham Lincoln Hogg. Abe dresses in all black and is a good man, unlike his brother. Boss gets Abe out of the way and tries to pull a scam where he's playing both of them until the Dukes show up with Abe and the only one who can tell them apart - Roscoe's dog, Flash, who only barks at Boss Hogg.
Yet another episode featured Boss Hogg hiring two thugs to impersonate Bo and Luke (wearing latex masks) and rob a bank or something like that.
In The Outer Limits episode "Replica", when confronted by his wife and a clone, a guy is able to tell them apart because the original had a tattoo.
Hercules: The Legendary Journeys gave us some heroic examples in their Other World episodes. In "Stranger in a Strange World," Iolaus has to masquerade as his cowardly jester counterpart after an accidental switch. Later in "Stranger and Stranger," Hercules briefly masquerades as the Sovereign to get information from Nebula 2. To ensure his success, he actually throws a man out a window (though he had a soft landing arranged).
In the Farscape episode "Exodus from Genesis," the aliens-of-the-week create voiceless duplicates of the crew. The real crew give themselves markings to try to spot imposters, but Crichton discovers that the duplicates can change their bodies to reflect such markings at will.
Shining Time Station: In "Mr. Conductor's Evil Twin", when Kara has to distinguish the two doubles. Remembering the real Mr. C's whistle was out of tune, she asks both to play a note.
In the Without A Trace episode, "Doppelgänger II", the team was hunting a serial killer who had a (completely innocent) twin brother. At the end of the episode, Jack and Sam come across "Greg" (the good twin) in an elevator, who claimed Rick (the bad twin) had run off. Jack spots what looks like blood coming from the ceiling tiles and calls out "Hey, Mouse!" Being called "Mouse" was Rick's Berserk Button, and his reaction tells them that they're facing Rick and that the blood is probably coming from Greg (who was still alive, but barely).
Crow: Okay, what condiment did I pour into your sneakers only two weeks ago?
Servo: Oh! Ah, uh...
Pod Servo: Was it... ketchup.
Servo: Damn, you are me! Gee, I guess I am a fraud...
When Mike asks what's Servo's confirmation name, Servo and his pod duplicate work together to answer the question, but the fake is finally outed when Mike asks about Servo's underwear collection.
In F Troop, bandit Kid Vicious has tied and gagged his good twin, Captain Parmenter, preventing him from shouting about the switch. However, as the sheriff prepares to take Parmenter away, the captain trips on the rug in a very familiar fashion. Sergeant O'Rourke has the gag removed, allowing Parmenter to prove his true identity.
The Bible makes this Older Than Feudalism. King Solomon was famous for his wisdom, not only because he prayed for it, but because he was able to figure out which of two women was the mother of a child (a trope of its own). One woman's baby boy was dead, and both women claimed to be the mother of the living baby. He spotted the imposter when he offered to take a sword and cut the baby in half, allowing each woman to have a half of the child. The woman who protested that decision by giving up her claim on the child was judged to be the true mother. 1 Kings 3:16-28
Also, likely the origin of the "kill both of us" gambit, though that is a variation.
One week-long arc in Dilbert had the Pointy-Haired Boss abducted by aliens and replaced by a double. Everyone in the office immediately realizes this because the double is competent and possesses people skills. Due to the differences being good ones, they don't even try to save the real boss (Who frees himself by accident when he teaches them Earth management techniques, causing the UFO to crash after the captain downsizes half the crew).
The Spy in Team Fortress 2 turns every game into one of these. You can't really be sure if that guy on your team is actually on your team, or if he's just waiting to stick a knife in your guts.
Or more likely, it turns every game into a case of "Kill 'em all, let the friendly fire system sort it out!" But servers with friendly fire enabled do play this trope straight, breaking spies in the process.
Done again in Super Paper Mario, where Mimi impersonates Merlee. You go through a dating game imitation to determine which one is the real one. The real way you tell them apart is by the fly hanging around the real Merlee - she was hiding in a toilet just before this incident.
It's also possible to tell by the way they speak - Merlee's rhymes and rhythm are perfect, but Mimi's are subtly inconsistent.
Also because Mimi's rhymes are about nasty and evil things.
In Kings Quest VII, near the end, the impostor king and real king fight each other as the hero stands by with a wand which can change the impostor back to his true form.
Tales Of Symphonia Dawn Of The New World.Done hilariously when Emil comes upon two Lloyds fighting each other. Even if you never played the original, it's still pretty easy to figure out which one is the Lloyd responsible for all the atrocities in the world — he's the one spouting the cliched lines about justice and bouncing on the balls of his feet with a real-life ":D" expression.
This is actually easier if you've played the original Tales of Symphonia, since you'll remember that Lloyd hates cheesy cliches about justice.
Averted in Fire Emblem: Radiant Dawn. Pelleas, the alleged son of King Ashnard, hardly resembles Ashnard's real son at all, except for a mark on his forehead. In fact, the identity of Ashnard's real son is essentially irrelevant to the story.
In Metal Gear Acid, Teliko gets doubled by La Clown and they have a game of this with Snake. Her cover is only blown when the game begins again and the player (controlling both Snake and Teliko) gets a turn as Teliko.
There's also one in Metal Gear Solid Mobile, when Snake starts receiving Codec calls from a mysterious figure. This figure later claims to be Otacon. But Snake is being helped out by Otacon already, and he knows nothing about the other person — and can't even detect their Codec calls. It's later revealed the interloper is the real Otacon. The Otacon helping Snake is an AI based on Otacon's personality in the hellish VR simulation Snake has been put into.
Live A Live has the player stuck in the "Prove yourself" half of this in Cube's chapter. When faced with both Cube and a hostile robot modeled after him, Kato muses aloud that since the fake was only activated a short time ago, he couldn't have heard Kato's first idea for Cube's name (Koro or Rover, depending on your translation; either way, it's a cliche dog name). When you give the right answer, Kato yells for Darth to shoot the other one.
During the Azure City War arc the Order can see three different figures who look like Xykon—two fighting, one hanging back on the enemy side—and have to figure out which the real one so they can stop him from getting to the Gate. Haley eventually figures that it's none of them—after all, if he can make two copies why not three? Then they realize the real Xykon is flying right above them on a zombie dragon.
Stickman: See, this is why you never, ever build exact robot duplicates of people.
Planet Zebeth has a sequence in which Samus fight against a backup copy of herself from a save file, and during the battle the scene cut away to other characters watching, so there is no way to tell who won. (However, the two characters are clearly not completely identical — they've experienced different things, so if there's ever a situation in which the differences come up we will know!)
Khrima, the main villain of Adventurers! made robot copies of the whole heroic party to replace them with, in one strip. The plan fell through since, after all, what's the point of replacing the WHOLE heroic group? He even made a robot copy of Spybot, who * was* a robot (and supposedly working for Khrima anyway).
In The Dragon Doctors, Elizabeth, disguised as Goro, uses the bond of enmity between them to hide her true form completely, even from Sarin; it would have worked, except that while Sarin indeed couldn't sense her true form, she couldn't sense Goro's true (male) form, either.
Played straight in The Gmod Idiot Box. Louis is trying to decide which Francis is real (one is a spy). He tells them to go up stairs. The one that gladly does gets shot. It then gets parodied the next moment, when Louis decides that they have to go up the stairs, and the real Francis hates stairs, so he shoots Louis.
Whateley Universe story "Test Tube Babies": Flying Brick Lancer fights power mimic Duplikat. Shroud finds herself facing two identical Lancers. One even tells her to shoot both of them to be sure. The team has a secret comm system so Shroud knows which one is the real Lancer.
If you shoot the one with the green shirt, both the remaining guy AND the girl are revealed to be aliens.
If you shoot the one with the yellow shirt, you killed the alien impostor.
And if you shoot neither, the woman instead suggests a threesome between herself, her husband, and the alien clone, which is then revealed to be just a nightmare that the real husband was having.
They left out the obvious fourthoption — shooting both of them.note Or if you wanted to get especially comedic, shootingherself. Or running off. Or shooting her two husbands and herself. Really, there are tons of other options besides those three, but shooting both of them really is the most obvious one they left out.
Happened in a third-season episode of Gargoyles. Where Proteus is disguised as Elisa, and Goliath asks them both if Elisa ever doubted him, even for a second. One Elisa claims that she would never doubt him... and that's the clue that proves to Goliath that the speaker is not Elisa.
One episode of Johnny Bravo has Johnny have to figure out which is his real mother, between her and an obviously robotic duplicate. Of course, being Johnny, he picks "the shiny one", though when the robot duplicate tries to terminate Johnny's mom, he pulls out its battery pack, stating that his real mom would never hurt another living thing, not even a robot (right after she just got in a stylized fight scene with said robot, and right before she starts smashing it with a hammer.)
Foster's Home for Imaginary Friends, "Bloo's Brothers": After trimming a bunch of impostors down to 2, Mac ID's his Bloo after the remaining impostor goes on and on about how much memories they've (supposedly) shared together.
Sappy Bloo: Mac, look me in the eyes, you know it's me. Think of all the good times we had together. The bond that we share, that only best friends can. Mac, you complete me. I-I love you. Mac: Okay, sappy, it's definitely not you. (points at other Bloo) You're the real Bloo. Bloo: Yes!
Codename Kids Next Door, "Op PRESIDENT": The 4th Grade President and a robot replica thereof fight on a bus on its way to the City Hall (the real pres wants to cut down on school hours). One of them gets thrown off. Nigel stops at a city dump to slag the fake with an electromagnet and car crusher, while Hoagie takes the thrown-off real president to the Hall himself...except that the actual president was then Brainwashed by recurring villain Father, and school hours end up being extended to 8:25pm. (Although it may not have been complete brainwashing, due to the President in Operation SNOWING being an out-and-out villain as well as giving a reason he turned evil.) In short, it didn't matter who got to City Hall Father was guaranteed to win.
Another example occurs in "Operation: P.O.O.L." Numbuh Four was clueless at first about Negative Numbuh One and Eizzil who were posing as Lizzie and the true Numbuh One; but, when the other members of the DNK showed up, he could tell that Negative Numbuh Three wasn't the real deal simply by looking at her. (As he told the imposter, "It's called being a friend!" Of course, his reasons likely went gone beyond simple friendship...)
Subverted on the early-90s X-Men animated series. Morph, a shapeshifter, attacks Wolverine, shifting into his form. Jubilee, who can fire energy blasts, enters as they're circling each other. One Wolverine tells her to shoot them both, so she shoots the other one. However, the first Wolverine is actually Morph, who flees. Jubilee says that she figured only the true Wolverine would say that, while Wolverine remarks that Morph had the same idea. (A What an Idiot moment, by the way, since Jubilee could easily have shot both of them; Wolverine's famous healing factor would — and, in fact, did — have him back on his feet and none the worse for wear in seconds. That, and this is Wolverine. Do you really think he'd actually say to shoot 'em both, bub?)
Wolverine probably would just because he KNOWS he'll get back up and the other guy won't.
1) Morph is not a villain! He's a Brainwashed and Crazy X-Man that Wolverine cares a lot about (by Wolverine standards). He's trying to capture Morph, so they can help him, and wouldn't actually want him hurt. Fortunately, 2) Jubilee's powers aren't all that strong, so she wasn't gonna seriously hurt anyone, healing factor or not.
South Park did the same thing, with the real (evil) Cartman correctly guessing that the "Shoot both of us" ploy would fool the others (who wanted to keep the "impostor").
Subverted in The SimpsonsTreehouse Of Horror; when an army of marauding Homer-clones attack, Marge stops them by leading them into a canyon. She returns to Homer, but finds out the one that survived was actually a clone; however, he starts giving her a backrub, and she decides she doesn't mind too much.
In a non-THOH ep., Homer becomes a local Krusty "helper" (i.e. dons the makeup and costume so as to look identical to the original for local publicity), but the original Krusty is in trouble with the Mob. Both end up cornered at gunpoint in an Abandoned Warehouse, where the villains are initially flummoxed as to which one to shoot (which partly lampshades the show's limited animation character styles as both Homer & Krusty share the exact same basic frame, as well as both being voiced by Dan Castellaneta, which Groening actually intended so as to create a dynamic where Bart hated Homer but worshiped a man who looked just like him in the original shorts, and one of the early shorts story ideas was the revelation that Homer was Krusty), until Homer blows their cover by addressing Krusty by name. This being comedy, they survive anyway. Despite the limited animation character style, it actually was quite easy to tell the two apart - Krusty has a tuft of green hair on the top of his head, Homer two single hairs combed over it.
Played with in an episode of Family Guy, during a Flashback to when Peter defeated his "evil twin". Lois has to pick one to shoot, shoots one, and while she and Peter are hugging, his face flips down showing he's a robot.
Parodied in the episode "German Guy", where Chris befriends an elderly puppeteer, Franz Gutentag but discovers that he used to be a Nazi. When Franz finds out, he takes Chris and Peter hostage; Peter knocks the gun out of his hand, and Chris grabs it, but suddenly he's confused which one to shoot. This is despite the fact that they look nothing alike, as Peter points out. Chris asks when is his birthday; Peter doesn't know, but Franz does, so Chris ends up shooting Peter in the shoulder.
It's used more straight in the next episode "The Hand that Rocks the Wheelchair", where Stewie creates an evil (or more evil) clone of himself, and Brian ends up pointing a gun at them; he lampshades the situation saying "Oh, come on, not this thing, really?" He eventually asks them to look at their feet, and shoots the one who doesn't start to laugh, as the real Stewie told him earlier that he spent an hour laughing at his feet. Unfortunately the Ambiguous Clone Ending suggests that Brian shot the wrong Stewie; it also helps that said ending is a Shout Out to Michael Jackson's Music video of "Thriller".
This is also lampshaded at the end of "Thanksgiving", in which the "real" Kevin Swanson suddenly appears and tries to warn about the imposter in the room. "Wait! That man's an imposter; I'm the real Kevin Swanson!" Peter then remarks "Guys I don't think we have time for this." right before the episode ends (Conclusion: there was No Ending).
Danny Phantom: Danny gets accidentally body-switched with a ghost and proves his identity by telling Sam about the time Tucker threw up in her lunchbox back in elementary school.
In Transformers Generation 1, the Decepticons built a fake Optimus Prime that was remote-controlled by Megatron. However, the fact that he was so unfamiliar with the Autobots themselves almost blew his cover right away. He called them by the wrong names (like addressing Ironhide as "Bumblebee") and acted like he barely knew them, causing them to quickly become suspicious. When the real Prime showed up, The Autobots came up with some really stupid tests (one of them was to have a race) and when they didn't work, Megatron tried to have the imposter gain their trust by creating a clone of Starscream and having the Prime clone kill it (knowing that an imposter likely wouldn't turn on the Decepticons' second-in-command. It almost worked... But Megatron proceeded to expose himself by not being particularly worried about where their human buddy Spike had disappeared off to (being a little too eager to lead the Autobots to their doom).
Another episode had some of the Autobots disguising themselves as the Stunticons, a group of car-based Decepticons. This in itself is already fairly impressive, since almost none of the Autobots in robot mode have physiques resembling the Decepticon they're copying. The Stunticons naturally manage to escape, leading to a situation where two teams of Stunticons have to oppose each other, culminating with both the real and fake Stunticons merging to form Menasor, with a little help from electromagnets and hologram devices in the case of the Autobots.
Subverted in the Spongebob Squarepants episode "Imitation Krabs", where SpongeBob can't distinguish between Mr. Krabs and Plankton's robotic replica of Krabs — despite the fact that one is clearly a robot, complete with Robospeak. SpongeBob asks questions that (he thinks) only the real Mr. Krabs would know. Krabs, however, fails to answer each one correctly, and is thrown out.
The Tick featured a classic episode playing up Spot the Impostor, where an evil tentacular alien makes a very bad clone of the Tick's sidekick Arthur to spy on him. When the real Arthur escapes and confronts the Tick, he becomes infuriated that the Tick can't tell which of them is real despite the fact that the clone is green in color, shuffles around like a zombie and can only say the phrase "I... Arthur" (to which the Tick replies "He's got a good argument there.").
Averted in an episode of Teenage Mutant Ninja Turtles 2003 involving a robotic Master Splinter. Hun begins to try this when the real Splinter shows up, only to be reprimanded by Shredder, who has already decided that they'll never fall for such an obvious trick.
Used with a bit of a twist in Jonny Quest The Real Adventures: In the "Questworld" virtual reality program, Jesse has two Jonnies at gunpoint. One starts reeling off the details of their last race in Questworld, at which point the other counters that, that's in the computer's memory banks, then brings up their real-life race immediately thereafter, at which point the double attacks him and gets zapped in about half a second.
DTZ, from the Chip N Dale Rescue Rangers episode "Dale Beside Himself", wasn't really evil, he just didn't want to go home, so he took Dale's place and sent him off to Fleeblebrox. Dale manages to come back, but the Fleeblebroxians aren't leaving Earth without him. With the threat of planetary atomization hanging over his head, Dale gets the idea to retrieve a platter of erkburgles (single-eyed squid-blob things with green skin and red scleras)... DTZ's favorite food. Unable to resist, DTZ starts scarfing and is subsequently caught.
The eponymous clown in the Garfield and Friends episode "Binky Goes Bad!" is jailed and brought to trial for the crimes of a man who hated him, and who decided to dress and make himself up like him in order to commit crimes and have Binky take the fall. Stinky is eventually revealed as being the fake when Garfield gets the judge to say "Order in the court," and Binky "orders" a ham on rye, hold the mayo. Stinky, completely failing to make a bad joke, or indeed, any joke, is sent off in the paddy wagon. Incidentally, Garfield is dismayed that he's in an evil twin episode.
This happens fairly frequently in Code Lyoko with the Polymorphic Clones.
The first occurrence was with XANA himself impersonating Jérémie in "Ghost Channel". (Basically, it was a case of Evil Cannot Comprehend Good; XANA insisted that Jérémie was too afraid to ever come to Lyoko, but the other knew that he would if their life was in danger. XANA's ignorance of how Jérémie would react led the others to realize it was him.)
In "XANA's Kiss", when confronted with two Jérémies, Aelita can guess which one is genuine because the Clone had kissed her without permission, which the true Jérémie would not have dared to do. (Maybe XANA can comprehend good, just not very well...) Later in the same episode, Odd recognizes the real Yumi from the Clone impersonating her when she calls him a dork.
Subverted in "Opening Act", when Jim Moralčs can't tell apart his nephew from the Clone, and ends up dumbly attacking the real Chris.
Both time that X.A.N.A. pretended to be Franz Hopper, he got caught by two different people. In "Franz Hopper", it's Jeremie realizing about line about Franz's ruined dairy. "Distant Memory", it's Aelita who realizes what's going on when X.A.N.A. calls " Skidbladnir" a funny name and no virtual take for Aelita's mom.
Subverted in "Image Problem". The team was suspicious of Yumi's imposter, but no-one truly caught on until she actually became hostile. (Of course, it was the first time XANA had tried something like this.)
In Darkwing Duck, they used the split personality variant for the episode "Negaduck". When Gosalyn and Launchpad decide to set free the good Darkwing to help them retrieve the Applied Phlebotinum that caused the split, in order to undo it, they mistakenly release the evil Darkwing, and he pretends to be the good one briefly, until he can effect his getaway. In subsequent episodes, the unsplit genuine Darkwing has an actual Evil Twin who goes by Negaduck, and on multiple occasions one of them has disguised himself as the other, resulting in Spot the Imposter scenes for the allies of whichever one they're disguised as.
One memorable moment has Darkwing and Negaduck dressed identically (except Negaduck wore a black mask) and acting so much like each other that they mirror each other's words and body motions perfectly. Then Darkwing breaks the impasse by declaring that a cute defenseless bunny rabbit is nearby, whereupon Negaduck (who was hatched from a Villain Ball) compulsively tries to kill the bunny with a shotgun. Before that, however, we get this memorable solution:
Dr. Bellum:(completely nonchalant) Well, we'll just have to kill them both. (arms bazooka)
Funnily enough, Darkwing tries the same ploy to infiltrate the Fearsome Five's HQ., using a Negaduck costume (minus his purple mask, again for audience benefit). He gets all the way up to the five — WHO HAVE JUST RESOLVED TO KILL NEGADUCK FOR SCREWING THEM OUT OF THEIR SHARE OF THE CITY'S LOOT.
Darkwing Duck: Hey! What's the deal? IS THIS ANY WAY TO TREAT YOUR LEADER!?!? (So much for the 'perfect disguise!)
When Negaduck returns, it leads to one of the best Never Say "Die" aversions ever seen.
Bushroot: Kill Negaduck! Kill Negaduck! Negaduck: (still reeling from Darkwing's anvil attack) Kill who?! Darkwing: No, wait, I'm DARKWING Duck; HE'S Negaduck! KILL HIM!! Negaduck: You thought he was me? You were going to kill... ME?!?!? Bushroot: No, no, no! We knew it was Darkwing! We were just testing, see? Quackerjack: Wait, didn't we already kill Darkwing Duck? Bushroot: Gah! THE GHOST OF DARKWING DUCK!! Liquidator: He's baaack! Negaduck: (beyond pissed) JUST... GET HIM NOW!!!!!!!
An episode of Duck Dodgers features an escaped criminal named Drake Darkstar who looks almost exactly like Dodgers. Dodgers tries to prove his identity to the police by appealing to the Cadet by listing things only he would know, but every single thing he lists is a mean prank he played on the Cadet. The Cadet ends up declaring Dodgers to be the criminal, reasoning that Darkstar would be nicer to him than Dodgers ever was.
Subverted in Stroker And Hoop when Hoop and his evil-identical-cousin are holding a gun on each other, with Stroker nearby. Stroker shoots one of the Hoops in the knee, causing the other one to exclaim "Good job, Stroker" before Stroker kneecaps him too. "Now we can figure out who's who at our leisure!"
This being Stroker and Hoop, they still get it wrong (at least until they spot the evil one pursing his hands with an evil grin as they prepare to drive off)
C.A.R.: We'd better double back and make sure.
In an episode of Where On Earth Is Carmen Sandiego, Carmen's henchman Sarah Bellum attempts to take over Carmen's organization, and a big part of this is disguising herself as Carmen. After she captures Zack and Ivy along with the real Carmen, Ivy quickly figures out how to tell the two apart: Sarah is left-handed (because the controls on the vehicle she uses are on the left side) but the real Carmen is not (she tosses her a tool from her cell - with her right hand - that they use to escape). After they are separated from Carmen briefly and later have to decide who is who, Ivy simply throws it to one of them, and the fact that she catches it with her left hand is enough proof that it's Sarah. (As for why they decide to nab her and not Carmen, well, since Carmen helped them escape, they owed her one.)
Kim Possible: When confronted with a shape-shifted Camille Leon, Kim can quickly identify the real Ron, but when it is Ron's turn in another episode, he picks the wrong one, to Kim's annoyance.
Not really. Kim had to rely on Camille being unable to duplicate Ron's ever-present pet, despite the fact that Camille stood in a pose that was obviously nothing like Ron's and utterly lacked Ron's Cloudcuckoolander personality. In the episode where Ron had to pick, Camille apparently worked on those flaws in her disguises, and Ron, having been trapped in a crate the entire episode, had stumbled onto the scene only moments earlier, and had absolutely no idea what was even going on. Even Monique, who had been touted as the one who got it right, wasn't actually able to distinguish between the two; she simply identified the disguised Camille's outfit as an expensive one more likely to be worn by spoiled heiress Camille than by the more practical Kim. *
Plus Monique had helped Kim pick out that outfit to disguise herself a few minutes earlier!
Jackie Chan Adventures: In an episode involving a group of Evil Knockoffs cloned off the Five-Man Band, Jackie's clone is found but the rest of the group needs to know which Jackie is the real one. Jade asks both Jackies if they can take her to an amusement park, the first says yes while the second says no and tells her to do her homework. Jade knows better and the first Jackie is exposed as the clone. In the same episode, Paco keeps calling Jade by pronouncing her name correctly, while in all the other episodes he pronounces it "Yade." She catches on to this as well and it is exposed that he too is a clone.
Before Jade asked about the amusement park, Capt. Black made an attempt by asking about his birthday. Neither Jackie knew the day.
Captain Black: Now that's just hurtful.
Wild Kratts: A Zachbot disguises itself as Chris in order to convince Martin that everything's okay, as opposed to, say, locked in a cell and glaring at his impostor from off-screen. Martin gets it in the end though.
In Transformers Animated, in order to determine who is the real Bumblebee after Wasp switched their paint jobs, helmets, and voices synthesizers, Bulkhead has them play a video game that Bumblebee is the undisputed master of. Wasp takes Bumblebee hostage before his cover gets blown.
Now, before that point, the real Bumblebee mentioned covering for Bulkhead back in boot camp (something no one else knew), but Wasp, who had looked up on the Autobots' files while the rest of them had gone, rendered that unable to work when they all got back to their base. At the same time, however, Wasp had proved himself completely pathetic at the aforementioned video game, hence why he cracked.
Futurama. The "Lesser of Two Evils" episode completely revolves around this trope, reaching the obvious climax when Bender and Flexo get into a Mirror Match. Leela has a laser gun but can't tell which to shoot, to which Fry unhelpfully remarks: "Flexo! Shoot Flexo!"
Subverted in yet another Mirror Match in "Rebirth", with Leela and her robot double fighting over Fry:
Leela: Shoot her! She's the robot!
Robot Leela: No! Shoot her! She's the human!
Fry: B-But how will I know who's the human and who's the robot?
Leela: We just told you!
Robot Leela: Yeah, you idiot!
Used to hilarious effect in a He-Man sketch on Robot Chicken. Skeletor commissions Beast Man to create a magical clone of He-Man. The result is the blue-skinned, incredibly stupid clone Faker. Undaunted, Skeletor sets Faker upon his enemies. Its odd appearance, mannerisms, and speech go unnoticed by all except Prince Adam himself; in fact, this laid back clone is far more popular and quickly becomes the life of the party, much to Skeletor's annoyance. When Adam returns as the real He-Man, Faker has either killed or captured most of his enemies. He-Man's outrage at the act, as well as the general hedonism Faker has inspired, leads the crowd to dub him a wet blanket and vote him as the imposter (even though Faker voted for him as the genuine article). He-Man is then executed, and it is only after Faker expresses no knowledge of being Prince Adam does someone finally catch on.
Alien 1: How where we going to take over the world with a white Michael Jackson anyway? Alien 2: Damn it! Damn it! Damn it!
Used in Action League Now on Nickelodeon. The villain has created a "perfect duplicate" of the heroic (if moronic) Flesh (who looks just like him except for the various bolts sticking out of every part of his body and his mechanically modulated voice). When the two Fleshes square off, the rest of the Action League try to determine which is the real one by asking for the League password. Both incorrectly guess "devilled egg". Finally Stinky Diver gets the idea to yell "Hey, stupid!", and the real Flesh responds.
Inverted in Chaotic (the cartoon based on the card game), when season Big Bad Aa'une goes One-Winged Angel, leaving Maxxor, Chaor, and Iparu hopelessly outclassed — until Iparu uses his copycat-shapeshifting power to mimic Aa'une's transformation and level the playing field! Chaor solves the inevitable who's-who dilemma by drawing attention to himself just before jumping off a cliff; Iparu instinctively saves him, and the good guys triple-team the real Aa'une.
In an episode of Richie Rich, an imposter impersonates Dollar, the dog. In an attempt to determine who is who, they present both dogs with a bowl of dog food. The imposter forces himself to eat it, and is promptly exposed. What he didn't realise was that the real Dollar is much too spoiled to eat common dog food.
Inspector Gadget once had to deal with this problem when Dr. Claw hired a Master of Disguise to impersonate him and spy at an important police conference. Gadget eventually confronts his evil lookalike, and no-one can figure out who's real and who's fake. Eventually, the real Gadget stands next to Chief Quimby, and his Gadget mallet activates on its own and bonks Quimby on the head. The dazed Chief immediately orders that the other guy be arrested, since the one standing next to him is obviously the real Gadget.
Johnny Test has this when Johnny and Dukey encounter their cyborg clones. After disposing of his clone with a "Fetch the Grenade" trick, Dukey takes the Clone's weapon and confronts the Johnnys. The cyborg immediately does the rapid spin clone switcheroo so they can't tell which one's real and which one's the cyborg. Dukey eventually manages to get the cyborg to give himself away by asking a math question, and blasting the one who answers immediately, knowing that Johnny always does math by counting on his fingers.
One episode of Goof Troop has Goofy and a counterfeiter who looks just like him. Max identifies his father by leaving an open bucket of wallpaper glue in front of the two of them and asking his real dad to step forward. Both of them step forward, but only the real Goofy steps in the glue.
In the Fish Police episode "The Two Gils", Gil's nemesis Calamari hires a guy named Bill who looks just like Gil, training him to act like him so that the real Gil will be blamed for whatever crimes he commits. Eventually the two meet face to face and get into a fight. No one can tell the difference, so they try a series of tests; nothing works. Finally Gil gets the idea of showing his badge. The fake reveals he has nothing on it, (the real Gil's contains a living starfish named Sandy), and the fake is arrested.
In the Mister T animated series, kid sidekick Spike and his older sister are led to believe that their eldest sister, who Spike idolized, has turned up alive, years after she disappeared in an accident. The well-planned scam is undone by Spike, when, seeing the fake of his dead sister on-screen, hears her call him Spike and ask for his help—except that Spike was a nickname he only picked up after she was gone.
Subverted in GI Joe Renegades when Zartan impersonates Duke. The Joes catch up and find the real Duke fighting the impostor, resorting to the usual Something Only They Would Say ploy by asking them a question about a conversation from the beginning of the episode. Duke doesn't remember it and they both say "no idea." Snake Eyes, who can hear the difference in their heartbeats, kicks Zartan in the face before it can escalate.
In Ozzy And Drix, Drix thinks up a rather unique way to do this when Ozzy is fighting his Evil Clone - he takes out his cellphone and dials the number for Ozzy's phone. The real Ozzy picks up the phone - Drix knew only the real one had it - and says, "Uh, I really can't talk right now!" And Drix replies, "That's okay, neither can I!" and then shoots the imposter.
One episode of Dog City has Bugsy Vile impersonating Barney Expy Bernie, the Big Pink Saint Bernard. At the end, the real Bernie appears and challenges his double to a sing-off of his Show Within a Show's theme song ('X is Good, X is Nice, X goes with everything, even with Rice', where X is something related to the theme of the episode). Halfway through verse three, Bugsy unmasks himself and begs the police to take him away.
On Regular Show a shapeshifting otter tries to take Rigby's place and the others can't tell them apart. Mordecai points at one of the Rigbys as the real one and asks for a hug. He does... which is how Mordecai knows he's the imposter, because the real Rigby hates to hug.
American Dad did this when Francine realizes that Stan assigned Bill (his double) to keep herself company while he dates the prom queen. She gets furious and pulls out Stan's gun, but can't tell them apart. One of them steps up apologizing and loves her, but Francine assumes the other Stan is the real one and shoots him thinking that Stan will never say something like this. It turns out the apologizing Stan was the real one and Bill was the one shot.
Spider Man The Animated Series: This happened in almost every episode that The Chameleon was in. And then there was the time that Spider-Carnage kidnapped and replaced our Peter (who was himself pretending to be this reality's—long story—Peter). Gwen Stacy sees through them both, recognises that our Peter is still the heroic one, and frees him.
Peter: How did you know that the other Spider-Man wasn't me?
In Transformers Prime, Bumblebee was able to figure out Nemesis Prime wasn't the real Optimus because Nemesis (and by extension MECH) couldn't understand Bumblebee's form of speech.
Another episode, introducing Wheeljack, has Bulkhead show misgivings about being with his fellow Wrecker. The Wheeljack that shows up is a shape-shifting body-copying Decepticon named Makeshift. Bulkhead forces 'Wheeljack' to tell a story about a battle, which Makeshift initially gets right... but he flubs one critically important detail, revealing the decepti(c)on.
Averted in the season 2 finale of My Little Pony: Friendship Is Magic when Twilight Sparkle and the real Cadance crash the wedding that has Queen Chrysalis posing as Cadance. The villain gives up the pretense concluding that since Twilight Sparkle's previous voiced suspicions about the bride now have the real Cadance's support, everyone else will instantly put one and one together and realize who is impostor is.
Played straight in the episode Too Many Pinkie Pies. Pinkie used a mirror pond clone herself, but too many clones were made and Twilight needed to know which Pinkie was real or she might get rid of her by accident. So Twilight forced all the Pinkies to watch paint dry and sent anyone who got distracted or stopped looking back to the mirror pond that they came from, because the real Pinkie would do anything for her friends.
The Life And Times Of Juniper Lee: Ray Ray had to identify the real Juniper. One failed attempt consisted on asking both Junes to tell her middle name. Both knew it was "Kim". Ray Ray eventually managed to tell who the real was the same way Jade did with Jackie in an above example. To her dismay, he was helped by a zit she tried to rid herself of earlier.
In the multi-part Justice League episode "Secret Society", it was pretty easy for Batman to tell that the guy claiming to be Flash was actually Clayface in disguise. As Batman put it, the villain "overplayed your part".
On another note, in the same multi-part episode, the villains were downright terrible at this. They didn't realize that J'onn J'onzz had disguised himself as Clayface until it was too late. Clearly, Grod is a sorry excuse for a villainous mastermind.
When Olive Oyl invited Popeye to eat hamburgers at her place, Wimpy disguised himself as Popeye and managed to fool Olive. Popeye then ate his spinach and defeated Wimpy. It's unclear if Popeye ever told Olive it was Wimpy or that he actually fooled her.
The cartoon version of Punky Brewster had the episode "Double Your Punky." While Punky is at a school picnic, Glomer creates a clone of her from a photograph to keep him company. But this clone is obnoxious and she escapes. In his confusion, Glomer zaps the sweet Punky back into the photo and tosses it away. He retrieves it and brings her back but during a scuffle with the clone, nobody can tell who the real Punky is—nobody except her dog Brandon, who recognized her scent.
The Beatles cartoon episode "No Reply" dealt with a jewel thief who has disguised himself as Paul McCartney. What tips him off is that when the boys are confronted by screaming female fans, the thief doesn't run.
The Too Dumb to Live citizens of Townsville take three hardened criminals poorly disguised as The Powerpuff Girls as the actual heroines in "Powerpuff Bluff." The real Powerpuffs get locked in jail.
"Him Diddle Riddle" poses a problem entailing two Miss Keanes, each bound and suspended over a tank of sharks. By asking them who they think the real Miss Keane is, Blossom figures it out. Though it also made for a pretty noticeable continuity error among fans of the series, as in spite of her reasoning being correct she still picked the wrong one.
Zig-zagged in the Danger Mouse episode "Penfold Transformed." DM has two Penfolds on his hands—a robot duplicate planted with him by evil Dr. Augustus Crumhorn and Baron Greenback's henchman Stiletto Mafioso in a Penfold suit. The real Penfold was kidnapped by Crumhorn, but he managed to escape and be reunited with DM after the robot—having been turned into a mechanoid designed to destroy our hero—is destroyed itself.
Boston landmark Faneuil Hall has an unusually-shaped weathervane (a grasshopper), which was sometimes used as a Spot the Impostor test during the American Revolutionary War.
Alan Turing proposed that a computer program which could win a text-only game of Spot the Imposter should be considered intelligent.
During the Battle of the Bulge, German troops in US Army uniform infiltrated the lines with the intent of causing sabotage and disruption. Checkpoints grilled GI's on things every true-blooded American was expected to know such as the identity of Mickey Mouse's girlfriend, baseball scores, or the capital of Illinois. This last question resulted in the brief detention of General Bradley; although he gave the correct answer (Springfield) the GI who questioned him apparently believed the capital was Chicago. Although this behaviour is often ridiculed in fictional depictions of the battle, it did succeed in identifying a number of infiltrators, many of whom didn't speak fluent English or know anything about American society.
Similar to the above, the parts of the French Resistance that helped Allied pilots back to England often quizzed the airmen they were helping. They often came across pilots who had knowledge of their unit, but none of their supposed home town, who were immediately detained as a possible mole. Once, they had a pilot who was the opposite — knew everything about his homeland, but nothing about his unit. Fortunately for him, one of the Resistance members was able to contact the RAF and established that he had been transferred there as a replacement from another unit, then immediately put on a bomber that was shot down over Europe.
One Allied pilot who was shot down and hidden in a barn by some locals found himself woken in the middle of the night by someone jabbing him in the gut and screaming at him in German. He reflexively protested in his native English, which served to prove to the French partisans that he was not a German spy. He reflected later on that it was fortunate that he did not reflexively reply in German, which he had studied in school.
There have been accounts of World War II German soldiers spotting careless spies at rallies because they saluted wrong.
Likewise, other careless slips, such as cutting and eating a steak the wrong way.
According to US Air Force tradition, the Challenge Coin became a tradition during World War I after an American pilot who had been shot down, captured by the Germans, then later escaped, having only a medallion with his unit's insignia engraved on it (his identification documents having been confiscated by the Germans). This at a time when it was common for infiltrators to don enemy uniforms to sneak into enemy camps and conduct recon and sabotage.