A Secret Identity is one of the most important secrets a costumed superhero can have, and the most carefully guarded. However, no matter how Crazy-Prepared a hero thinks he is for any given contingency, eventually someone — for purposes of this article, let's call him "Tucker" — will stumble onto his secret. So how can one convince Tucker that he is mistaken, that the superhero and the civilian are not one and the same?
How about seeing the hero and the civilian in the same place, at the same time?
Taking advantage of the fact that most superheroes wear a face-concealing mask, it is easy to slip a third party (usually a close friend or confidante) into the hero's costume, then have the actual hero appear next to the costumed person, in civilian clothing, in the sight of Tucker. Seeing the civilian and the hero standing side by side is usually enough to convince Tucker that he was mistaken, and the Secret Identity is safe.
The ruse can become more convoluted and more convincing if the hero has superhuman powers which the costumed decoy must duplicate, if his disguise doesn't involve a mask, or if the decoy is an unwitting dupe, but a clever hero can usually get around this.
Of course, if you're an Anti-Hero or a Villain, you could always just kill Tucker.
Compare Dead Person Impersonation, see also Conveniently Unverifiable Cover Story.
Not to be confused with Costume Copycat, a person who impersonates the hero without the hero's knowledge or permission.
As Ran often comes close to discovering Conan's identity in Detective Conan, Conan has found a couple of ways to have "Shinichi" on the phone when Conan is in the room. The masterstroke came one time when he was in his original body and Ai disguised herself as Conan.
In Sailor Moon, after Kaolinite discovers Usagi's secret identity, Minako uses Usagi's disguise pen to pose as Sailor Moon. In this form, she uses her own powers (being Sailor Venus), but wisely avoids calling them by name... sort of... Hissatsu Love-Me Moon Chain compared to Venus Love-Me Chain...
It gets even weirder in the R season when it's obvious that Moonlight Knight is Endymion/Mamoru .... until the two are seen together. This confused, pretty much everyone, including undoubtedly the viewers until it's revealed that Moonlight Knight is the SOUL of Mamoru who can ... for some reason ... leave his body and create a dublicate without his knowledge.
Kaitou Saint Tail goes through an elaborate plan involving Seira dressing up as Meimi while Saint Tail's exposing plagiarism in a contest entry, and then a Saint Tail-shaped balloon flying off into the distance while Meimi's out on stage. However, despite their best efforts, Rina still continues to believe that Saint Tail and Meimi are one and the same, so they try it again... dressing up a clueless, older, male guard as Saint Tail. This not only works but succeeds in breaking Rina's brain deeply enough that she swears off detective work forever.
In Code Geass, C.C. occasionally dons the Zero costume to cover for Lelouch. Fridge Logic kicks in when you realize that Lelouch is a half a foot taller than C.C. and has a completely different body type. In the second season, ninja maid Sayoko uses an almost frighteningly Latex Perfection to pose as Lelouch at school while the real Lulu is in China as Zero.
C.C.? Platform shoes, boob bindings, and control-top pantyhose. Not that hard.
Lelouch even did this all by himself once, using only a tape recorder, a phone, and his Geass.
Superman had a bevy of robotic lookalikes back then; it got to the point where everyone knew he had them, and he'd have to prove that he wasn't using a robot this time.
One time, the fake-Clark Kent was actually John F. Kennedy. Unfortunately, the comic showing this was released barely before JFK was assassinated, which created the uncomfortable connotation that Superman knew that the president would take his knowledge of his secret identity to his grave.
On another occasion, Superman appeared as himself, and Batman (Master of Disguise) appeared as Clark Kent; they even managed to get their picture taken together.
In current continuity, where it's less believable that Batman could wear a Latex Perfection Superman-mask over his cowl, the task usually falls to the shapeshifting Martian Manhunter, who can also duplicate Superman's powers, if the real version is being Clark.
Lampshade Hanging in one comic, where a special appearance isn't required; the mere fact that Superman and Clark Kent have been photographed together so often is enough for the characters to dismiss the idea.
In one Silver Age example, Clark fakes out the Daily Planet staff by using a life-sized balloon of Superman and "super-ventriloquism".
Superman will also take Batman's place from time to time. He has an easier job of it, since the cowl is so face-concealing and the two have similar builds. Amusingly, Batman has everyone in Gotham so psyched out that Superman-as-Batman can fly, or take bullets without flinching, or move at superspeed, without anybody thinking that maybe Batman is being impersonated because they think Batman does those things all the time.
Not a complete example, but it has been mentioned a few times that Alfred can imitate Bruce Wayne's voice perfectly, allowing him to do a phone conference or make an excuse for missing a date while the real Bruce is already beating people up in his underoos. In the old Batman TV series, Alfred put on the bat-suit a couple of times when Batman and Bruce Wayne had to be in the same place at the same time. No one seemed to notice his moustache.
Superman appeared on live television as Clark Kent and Superman at the same time, interviewing himself. He did it by switching costumes back and forth many times per second, so he would appear in both places each time the camera shot a frame.
The various Flashes have used the same trick a few times.
Post-Death of Superman, Supes rescued Clark from his presumably weeks/months of being buried under rubble from the Doomsday fight. "Clark" in this case was the shape-shifter Matrix.
A subtler bit of brilliance from this incident was the situation they put Clark in, making sure it was a situation where Superman was the only person likely to find him, to help explain the coincidental return of both Superman and Clark. It also explained why Clark's hair was grown out like Superman's and why he chose to keep it that way.
One strange example came when Batman was briefly dead; Hush, the villain who devoted himself to ruining Bruce Wayne/Batman, gave himself plastic surgery to look like Bruce Wayne and take the reins of Wayne Enterprises, with the intent of ruining Bruce's reputation and everything he'd worked for. The rest of the Bat-Family, however, managed to blackmail him into actually "sticking to the script."
Spider-Man has done this as well, most notably in a Silver Age story when he had the Prowler dress in his costume to convince his friends that an earlier admission of identity was just a result of feverish delusions. This came back to bite him, when George Stacy, one of the friends in the group, was killed during a battle between Spidey and Doc Ock, the Prowler came to the conclusion that the incident had been a setup.
Additionally, Daredevil and Spider-Man have impersonated each other successfully given the similar build and acrobatics.
Iron Man had to do this a lot, actually, mainly due to his cover story that Iron Man was Tony Stark's employee and bodyguard.
Ultimate Iron Man has Tony and his friend pretending to be robots, going so far as to let Tony's arm get cut off by shrapnel so that the people they were trying to trick would believe it even more. Fortunately, in the Ultimate universe, Tony Stark has amazing healing powers.
Barbara Gordon pulled this when her alter ego had to be introduced to her father in Batgirl: Year One. She made Robin (then Dick Grayson) wear her costume.
A plot thread in early Daredevil comics involved Spider-Man assuming Foggy Nelson was Daredevil (because he'd never have guessed it would have been the blind guy next to him), and Foggy consequently trying to keep up the ruse so his friend Karen Page would warm up to him romantically. Things actually managed to continue to get more complicated after that. Matt didn't want Foggy to embarrass himself, but he couldn't stop him without revealing he was really Daredevil.
Daniel Rand took over as Daredevil for a while when Matt Murdock was in prison. As this happened during the Civil War story arc, it led to a bit of confusion (and parody) as to which Daredevil it was supposed to be in the main arc of that storyline.
In the NOW Comics The Green Hornet universe, a supervillain called Johnny Dollar captures and unmasks the Hornet and attempts to auction off his secret identity. Unfortunately for him, in this universe the Hornet is a Legacy Character, and the previous Hornet comes to the rescue, showing up at the auction in the guise of the current Hornet, and claiming that the guy Dollar has is just an innocent bystander. The auction attendees are not amused.
In yet another Superman example, an episode of Lois and Clark had Clark's mom suddenly develop an interest in hologram technology, just in time to project an image of Superman while Clark is announcing the claim he's the Man of Steel is ludicrous. A later episode had a Clark from an Alternate Universe covering when Tempus spilled the beans, to his great embarrassment. "Come on! He's obviously from a parallel universe!"
In a variation on the usual Superman formula, one episode of Smallville sees Clark dressed up as the Green Arrow in order to help Oliver Queen 'prove' to Lois that he and Green Arrow are two different people. Lois (who was dating Oliver at the time) plants a kiss on her hero — only to realise her mistake as Oliver appears. Oliver would later repay the favour to Clark when Jimmy Olsen put two-and-two together. However, Jimmy didn't kiss him.
In another Batman example, Adam West's Bruce Wayne would occasionally be seen in public at the same time as his Batman... as played by Alfred in a backup Batsuit.
No, that one was a Bruce Wayne & Batman appearing together moment. Bruce gave Alfred a device so that when he spoke, it would come out in Batman's voice. Alfred told Gordon from across the street he had a cold and didn't want to contaminate anyone, which Gordon dismissed as "That's Batman for you - always thinking of his fellow man."
Barry Allen used a variation in The Flash television show; he managed to confuse his assistant Julio, who was just about to clue in, by using his super-speed to make a brief appearance as the Flash and then be right back in place as Barry when Julio looked back. (You wouldn't think this would fool anyone who was already suspicious, but apparently it worked.)
Zorro used this trick in the Disney TV series; when Capitan Monasterio accused Diego de la Vega of being Zorro, Diego's assistant Bernardo appeared disguised in Zorro's costume to throw him off the track.
Used by George/Thermoman in an episode of My Hero: Thermoman was scheduled to give a public appearance in Northolt and Janet's father threatened to cut off all ties with George if he didn't personally thank Thermoman for having saved Janet's life (his and Janet's first meeting). George made Tyler wear the costume then erased his memory.
Though not exactly the same thing, in an episode of The Andy Griffith Show, Barney is threatened by a local man who says he's going to beat Barney up if he ever sees him out of uniform. Andy convinces Barney's judo instructor to don Barney's suit and walk down a street where the man expects Barney to be walking late at night. The end result is that the man mistakes Barney's judo instructor for Barney, takes a serious beating from the judo instructor, and no longer taunts Barney.
In an episode of Nightman, a reporter becomes suspicious of Johnny, thinking (correctly) that he is Nightman. She tells Johnny that Nightman promised to give her an interview at a certain time. As Johnny tries to come up with an excuse to leave, she calls him out on this, only for him to point up at a flying figure. The reporter realizes her mistake. The viewers then see that it is Johnny's best friend Ralleigh who is wearing the suit, although he is not very adept at using the anti-grav belt.
In order to find his "missing memory", which he believes is the key to figuring out who the JunctionPoint is, Ryotaro Nogami a.k.a. Kamen Rider Den-O impersonates himself in the past, deceiving his sister, the one who knows who the Junction Point is, in that time period.
On Arrow Oliver Queen is the vigilante the police refer to as "The Hood". When Oliver is arrested for being The Hood, his bodyguard Diggle puts on the costume and beats up some bad guys. The police have to release Oliver and apologize to him.
WCW did this in the late-80's as the blow-off for the Midnight Rider angle. The Midnight Rider was Dusty Rhodes' masked alter ego and Dusty was slated to be in the Midnight Rider's corner for a match. That night, the Midnight Rider was portrayed by jobber The Italian Stallion (who didn't look anything like Dusty).
In Ace Attorney Trials and Tribulations Phoenix clears Ron of theft charges by showing that he was in a completely different part of town when Mask*DeMasque was caught by Luke Atmey's security camera. This wasn't Ron's plot, but Luke Atmey's. He dressed as Mask*DeMasque and had himself recorded well in advance, faking the time stamp on the footage, as part of his plan to be arrested for being Mask*DeMasque—and thus have an alibi for his real crime.
The regular cast of El Goonish Shive includes a shapeshifter, so when another character becomes a costumed vigilante, this is suggested to throw off suspicion. It's rejected for multiple reasons, most of them having to do with the shapeshifter's eccentricity.
A first season episode of The Batman had Bruce Wayne being interviewed on TV, and Alfred appeared on the top of Wayne Manor in the cape and cowl portion of Batman's suit. Luckily it was a night, it was raining, and the roof was 6 stories above the ground.
More Superman! A clone (who later became Bizarro) rescued Clark Kent from falling to his death, right in front of Lois.
In another instance, Superman went missing and Supergirl wound up controlling several Superman-robots to hide the disappearance. Lex Luthor saw right through the ruse and built a remote to assert control over them.
One episode has Superman filling in for Batman in Gotham City (Bruce has gone missing, and Robin can't fight the crime of Gotham alone and look for Wayne.)
The culprit was Brainiac, who kidnapped Bruce as the CEO of Wayne Enterprises, not realizing he was Batman. It helps that Bruce and Clark are of the same build, and that Superman's precise muscle control allows him to modulate his voice to match Batman.
Done with an illusion of Static in Static Shock; provided by a reformed villain.
Spoofed on The Simpsons, with Homer's secret identity "Pieman". He tells Marge that he is not Pieman, and to prove it, he shows a picture of himself with Santa's Little Helper (a dog) in the Pieman costume.
In Zorro Generation Z, Bernardo also impersonates Zorro. Of course, as the designer of this futuristic Zorro's bag of tricks, he actually doesn't make a half bad superhero himself. Of course, he had to work around the fact that, as always, he can't talk.
Done unintentionally in Big Guy And Rusty The Boy Robot, where Big Guy is in reality not a robot but a Humongous Mecha piloted by a man claiming to be an engineer. His sidekick Rusty does not know this. In one episode, Big Guy is infested with runaway nanobots, who proceed to repair the AI systems that were originally installed into Big Guy but replaced with a human when they did not work. Big Guy ejects the pilot and begins acting on its own with the pilot taking the sidelines. Unfortunately, the nanobots' constant repairs result in advanced wear-and-tear, causing Big Guy's AI to deteriorate.
In the Batman The Brave And The Bold episode "Triumverate of Terror!", Lex Luthor teleports into the Batcave, while Bruce is at the Bat-Computer sans mask. Luthor doesn't get a look at his face, but starts making an analysis based on hair colour and general appearance ... before a costumed Alfred asks "Are you here to fight me, or my caretaker?"
Alfred also poses as Batman in one episode of The Batman, when a TV reporter doing a feature on Bruce Wayne gets suspicious because Batman was defending Bruce from the Penguin, but she never saw them together.
Happens repeatedly during the My Little Pony: Friendship is Magic episode "The Mysterious Mare-Do-Well". Although Mare-Do-Well is actually four different ponies, so there's no actual "impersonation" going on, but they achieve the same effect by passing around the role and appearing out of costume to throw off the scent.
Yogis Space Race: Somehow, Captain Good/Phantom Phink never needed one to preserve the masquerade despite "Phantom Phink" never showing up at the start line when the races begin.
This actually was used in real life by Andy Kaufman when it came to his alter ego Tony Clifton. Andy always insisted that Tony was not him, and at the end of his 1979 Carnegie Hall performance they performed together on the same stage, after Tony had served as the opening act. (Andy played Tony for the opening, then had his brother play Tony at the end.) Andy later handed off the role to friend Bob Zmuda, who did concert engagements and TV appearances as Tony with most people assuming it was Andy under the costume...in part because Andy was fond of going to the concert venues and making sure he was seen during the day to further the ruse.
Tony's even made a few public appearances after Andy's death. Beat THAT, Superman!
Fred Dannay and Manfred Lee co-wrote mystery novels under two different composite pseudonyms: Ellery Queen and Barnaby Ross, whom they pretended were literary rivals. Dannay and Lee went so far as to stage public debates where one of them would play the part of Queen and the other would play Ross.