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 [[ConfrontingYourImposter 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 [[NoDelaysForTheWicked 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.


[[folder:Comic Books]]
* ''ComicBook/BatmanTheDarkKnightReturns:'' 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 ''Film/The6thDay'', 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.
* Multiple people in ''Film/TheFifthElement'' try to check in with Korben Dallas' identity. It becomes something of a RunningGag.
** Actually, Cornelius does it smartly. 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, 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 ''Film/{{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.
* ''Film/{{Ultraviolet}}'': 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 ''Film/LastActionHero''. 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 ''Film/FromParisWithLove'', the assistant to the American representative at the conference gets this, even though he's the real guy. The person who used his ID before was [[spoiler:his fiancée, the terrorist who tries to blow the conference up]].
* In ''Anime/TheCastleOfCagliostro'', 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.
* In ''Film/BlackKnight'', Creator/MartinLawrence's character Jamal is a FishOutOfTemporalWater 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).

* In the ''Literature/OutlanderLeander'' series, Leander enters the city of Cerna, sneaks 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.

[[folder:Live Action TV]]
* Used in ''Series/{{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 ''Series/MadAboutYou'', 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 ''Series/TheLastEnemy'', 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 ''Series/{{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.

[[folder:Western Animation]]
* In the ''WesternAnimation/LeagueOfSuperEvil'', this went along with the TotemPoleTrench gag.
* ''WesternAnimation/FamilyGuy'': 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.

[[folder:Video Games]]
* A variant is used in ''VideoGame/TheLegendOfZeldaMajorasMask''. 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 Link[[note]][[HelloInsertNameHere or whatever you named the hero]][[/note]] who made the reservation. You just happened to arrive before he did. Steal his room at your peril, [[VideoGameCrueltyPotential '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 VerbalTic 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 [[VideoGameRemake 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 ''VisualNovel/HotelDuskRoom215''. 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.
-> [[spoiler:[[FridgeHorror You have always been checked in.]]]]