In reality, identity verifications are designed so that one and only one person has one unique identity. Some people just don't know about this when forging them.
A character arrives at a security checkpoint where he has to verify his identity in order to gain entrance. He takes out his ID card, runs it through the scanner and is surprised to find that it is being denied. The receptionist then tells him that he is already checked in. This can only mean one thing: An impostor is on the loose! Curiously, the guards never seem to suspect the second person/group of being the true impostor.
If the hero is the impostor, it means that his cover is blown, and he must prepare to fight the local security team. If the villain is the impostor, chances are that he has already fulfilled his objectives and destroyed the evidence/killed the witness/sabotaged the equipment, etc.
A variation of this is when the security check is especially high-tech and demands a finger/hand/iris scan. In these cases, expect cloning or time travel to be involved.
- In The Castle of Cagliostro, Lupin disguises himself as Zenigata, and insists that Gustav (Cagliostro's Captain of the Guard) was foolish for not recognizing the previous Inspector Zenigata as The Wolf in disguise.
- Batman: The Dark Knight Returns: A detective shows up at a crime scene and talks to Police Chief Yindel. Then he walks away. Then he shows up, coming from the other direction. Yindel points at the departing cop and goes "stop that man!". The next panel is him leaping onto the Batcycle.
- In The 6th Day, Adam Gibson uses a finger from a dead clone to gain entrance. He is discovered when a new clone with the same fingerprint tries to check in.
- The Fifth Element: Multiple people try to check in with Korben Dallas' identity. It becomes something of a Running Gag. Cornelius does it smartly, as he creates a multipass in the name of Korben David Dallas for his assistant David. The spaceport people have no idea what Dallas looks like or what his full name is. Thus, David can still respond to his given name and not look out of place. In a blink-and-you'll-miss-it moment, the shape-changing Mangalore boss tries to use the face of Dallas's neighbor, who had been picked up by the police when they mistook him for Dallas; it doesn't end well for him (we can also thank Cornelius for that, as he was the one who switched the door tags before the police came to arrest Dallas).
- Subverted in Sneakers. Werner Brandes (who has a genius-level 180 IQ) knows that someone has stolen his ID card with the intent of breaking into the place where he works, and there's a (repeatedly shown) paper printout of who has entered and left the building. However, when he gets there he doesn't ask the security guards to check the entry/exit register and see if "he" is listed as still being inside the building, which would prove he's right about the break-in.
- Ultraviolet (2006): After Violet pretends to be a courier in order to sneak into the Arch-Ministry building, the real courier shows up and her ID is rejected, thus alerting the guards.
- Sort of happens in Last Action Hero. The fictional villain "The Ripper" shows up to a movie premiere (of a later movie in the same series) and is briefly interviewed by a reporter, who thinks it's Tom Noonan (the real-life actor) dressed up as the villain. Then, the real Tom Noonan appears, dressed as himself, confusing the heck out of the reporter.
- In From Paris with Love, the assistant to the American representative at the conference gets stopped at the checkpoint for being already checked in, even though he's the real guy. The person who used his ID before was his fiancée, the terrorist who tries to blow the conference up.
- In Black Knight (2001), Martin Lawrence's character Jamal is a Fish out of Temporal Water who hasn't yet realized he's in Medieval England. He assumes that the castle is a rival theme park and, when asked by the guards where he's from, replies that he's from Florence and Normandie (an intersection in South Central). They immediately assume he's the messenger from the Duke of Normandy whom they have been expecting. After figuring out that he really is in the past, Jamal continues the ruse to avoid being beheaded. Then the real messenger arrives a few days later, and Jamal is discovered in bed with the princess. Not only is his ruse exposed, but his tryst costs the king his alliance with the Duke of Normandy, who was expected to marry the princess (who must be a virgin, of course, although Jamal claims that she already wasn't).
- Captive Prince: While incognito in an enemy country, Prince Laurent goes to an inn as "Charls, the Veretian Cloth Merchant". The innkeeper disagrees and tells him that Charls is already there. Played for Laughs when the real Charls comes out to confront him, recognizes his Prince, and starts supporting Laurent's lie.
Charls: That's right. He is Charls. I am Charls. We are cousins, named after our grandfather. Charls.
- In the Outlander Leander series, Leander enters the city of Cerna, sneaks out without checking out, and later attempts to re-enter from the other side. In this particular case there was no impostor; he tried to check himself in twice.
- Used in Supernatural: The brothers check into an evidence warehouse using Homeland Security IDs, but are forced to flee once some very serious suits from Homeland Security arrive at the desk.
- A comedy variant: On Mad About You, Jaimie was going back to school, but Paul had forgotten to send in her registration forms. So he, Ira and Jaimie's friend Fran stealth sign her up for all the classes she wanted on the last day of registration, getting signatures from the teachers of the classes in question, and all had to get photo IDs taken, claiming they were Jaimie Buckman.
- Happens in the miniseries The Last Enemy, when the protagonist needs to talk to somebody being held in a secure medical facility. A fingerprint scanner is involved (but no cloning or time travel — it's pointed out that current fingerprint scanners really aren't that hard to fool).
- The first time the Leverage crew pretend to be FBI agents, they resolve the situation (and have the mark arrested) before finding out that the real Feds are on the way. To avoid this trope, they quickly tell the cops that their associates are about to arrive and take off. The real agents are a little confused but decide to play along and accept the glory.
- The X-Files: In the episode "Synchrony", Mulder is helping a scientist whose much older future self has travelled back in time to prevent his past self from causing the terrible future he came from (though the scientist is understandably very skeptical about the whole story). When Mulder and the scientist goes to investigate the laboratory the scientist works at, the scientist checks in at security via a hand-print scanner, and the guard tells him that he has already logged in some time ago. The scientist is of course confused by this, but Mulder quickly puts two and two together and realizes the man's future self is already in the building.
- Happens on occasion in Doctor Who, since the Doctor's modus operandi in a new situation is to act like the most important person in the room and let everyone else figure out who that would be.
- A variant is used in The Legend of Zelda: Majora's Mask. Link is shocked to discover that a room at the Stock Pot Inn has already been reserved for him. Turns out there's a Goron also named Linknote who made the reservation. You just happened to arrive before he did. Steal his room at your peril, 'cause you can find him outside shivering in the cold later if do. Remember that this is the festival season, and all the hotels were booked solid a long time ago. The Goron doesn't get angry if you steal his room, though: if you do so, his Verbal Tic makes him say his name as "Link-goro!". The receptionist doesn't understand him and sends him away saying there's no reservation under that name. In the 3DS remake, the receptionist remembers that a Goron made the reservation, and you can't steal the room until you have the Goron Mask.
- Subverted in Hotel Dusk: Room 215. Protagonist Kyle Hyde books a room at Hotel Dusk, and when he signs his name the receptionist asks "Really? Is that really your name?" After a little bit, it's revealed someone with the same name stayed at the hotel the year before. Naturally, Kyle gets suspicious.
- Call of Duty: Black Ops II has a variation in versions of the ending where Alex Mason survives Panama. David Mason checks in with Woods at the Vault, only to be asked by the guard out front "another Mason?" David immediately assumes something's wrong and tells him to secure the building, enters Woods' room, and finds nothing out of the ordinary other than a new guest who turns out to be his long-thought-dead father.
- In the Skin Horse storyline "Swiftly I Glide", Tip, Nick, Unity and Dr Lee are infiltrating an Anasigma base, something that turns out to be easier than they expected because the base was expecting an inspection team. They manage to blag their way through this quite well, until:
Gail: We have a little problem.
Tip: Don't worry, the Anasigma inspection team is trained to handle the most unlikely disasters. So what's the problem?
Gail: The Anasigma inspection team is at the gate.
Unity: What about the most likely disasters?
Tip: Those, not so much.
- In the League of Super Evil, this went along with the Totem Pole Trench gag.
- Family Guy:
- Meg's getting married, Neil Goldman isn't invited but wants in. Chris is checking at the door. After giving his real name and being rejected, Neil says he's Chris, who immediately switches places without question. When Chris gives Neil his name, he's told Chris has already checked in.
- Another episode includes a slight variant. Peter impersonates Mel Gibson to gain access to Gibson's vacant reserved hotel room, but when checking out he is overheard by members of Gibson's entourage who are there to check in for him. A chase scene results.
- In the Episode "Roads to Vegas," Brian wins tickets to see Celine Dion in Vegas, and takes Stewie along. However, their attempt to get to Vegas via teleportation results in copies of them appearing in Vegas. The copies of them have no trouble checking into the hotel and going to the Celine Dion concert, but the original's efforts fail because they've "already checked in to the hotel" and their tickets to the concert were already scanned.
- In the Young Justice episode "Satisfaction" the renegade Speedy (original Roy, soon to be Arsenal) has gone to one of Green Arrow's secret bases to get the equipment he needs to kill Lex Luthor. The base uses a retinal scanner to verify authorized users. Soon after Green Arrow and Red Arrow (clone Roy) arrive looking for him. Upon scanning Red Arrow's eye the computer says "Error. Red Arrow already inside. Security breach. Lockout." Green Arrow is able to override the lockout, but this is how they find out where Speedy is.
- Subverted in Bounty Hamster. While Marion is impersonating a famous hotel critic as part of a Zany Scheme, the real critic turns up at the front desk, demanding his usual room...but the steward is away from the desk at that precise moment, so Marion grabs the critic, stuffs him in a cupboard, and nails the door shut.