In most time travel/cloning/alternate reality stories, one character can have multiple copies of themselves running around in the same time period. Some [[NeverTheSelvesShallMeet might have problems if those copies meet each other]]. Then there are these cases...

Only One Me Allowed Right Now is a case where the universe either flat out denies multiple copies of a character to exist in a same time period, or that either the character and the copies go crazy or the universe [[RealityBreakingParadox starts to break down]], or something bad happens.

Note that this is NOT NeverTheSelvesShallMeet. In that one, you can have millions of copies of a same character running around in the same universe without that much trouble, only they must not meet each other. In this case, even though there are only two copies and they are at opposite ends of the universe, the problem still happens. Not as tightly related to the OneSteveLimit as you might think.
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!!Examples:

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[[folder:Anime and Manga]]
* In one episode of ''SgtFrog'', Keroro accidentally [[MesACrowd clones himself a thousandfold]] using the Kero Ball. After various hijinks, the clones start to fade. Kululu gives a {{Technobabble}} explanation, but when told by Natsumi to "use words real people understand," he gets down to the point: if they don't destroy all the clones, then the Kero Ball will overload and all the Keroros, including the original, will disappear.
* This turns out to be a major plot point in ''{{xxxHoLic}}''; [[spoiler: the main character]] is a time travel duplicate. As a result, he hates himself and draws in supernatural beings trying to grant his wish and kill him, and if he doesn't develop strong social connections he'll soon cease to exist as reality corrects itself.
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[[folder:Comic Books]]
* This applied to the DCUniverse [[PreCrisis before the first Crisis]]. If a character traveled to a time where he or she already existed, it (the version not belonging to that time) would become an invisible, unheard, ineffectual phantom until it stepped out of that moment in time. Note that back then, even if characters were allowed to "meet themselves" [[HitlersTimeTravelExemptionAct history could ''not'' be changed,]] so it was pointless anyway.
** Franchise/{{Superman}}'s childhood friend Pete Ross, of all people, found a way around this. He was [[FightingYourFriend furious at Superman]] at the time (he blamed Superman for his son getting kidnapped by aliens; long story), and wanted a way to fight him. So he got ahold of some [[AppliedPhlebotinum phlebotinum]] that let him [[BodySnatcher swap minds]] with Supes's younger self, ComicBook/{{Superboy}}, and then returned to the present in Superboy's body, to duke it out with adult Supes. It turns out the [[OnlyOneMeAllowedRightNow Only One Me]] rule only applies to your ''mind'' being in two places at once, not your body. This actually makes a degree of sense, since even if you travel back to before your birth or after your death, the matter that makes up your body should still be around somewhere, and yet you only appear as a phantom iif you travel within your own lifetime.
** Some early SilverAge Franchise/{{Superman}} stories (& one [[JusticeLeagueOfAmerica Justice League]] tale) used the idea that if Superman traveled within his own lifetime the earlier version would take his place in the present. So if Superman traveled to when he was Superbaby, Superbaby would appear in the present while Superman was in the past.
* In ''ComicBook/NthManTheUltimateNinja'', it is impossible for a person to be in two places at once. This causes one character to spontaneously combust when she returns to the past, as she arrives at the same time she's being born.
* This happened to Jubilee in an issue of ''{{Wolverine}}'': In her youth, there was an incident where she was in a car with her friends, who suddenly asked her why she momentarily disappeared into thin air. Not remembering doing so, she dismisses it as her friends acting crazy. Years later, she briefly falls into the time portal belonging to Gateway, appearing in her parents' house, at the exact same time she "disappeared" in her friends' car. It is explained that two of her couldn't exist in the same point in time, so her younger self simply vanished until the older version returned to the present.
** Normally, time travel doesn't exactly work in the Marvel Universe. If you travel into the past, you end up in the past of a similar but distinct universe (which you might not be able to distinguish at the time). For example, when Rachel Summers (daughter of Jean Grey and Scott Summers) traveled into the past, she ended up in the primary Marvel Universe instead of the offshoot where she was born (where Jean Grey was depowered instead of killed). She didn't realize she wasn't in her own timeline until she saw Jean Grey and Scott Summers had a son... she never had a brother.
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[[folder:Fan Works]]
* In ''Fanfic/GeorgeWeasleyAndTheComputationalError'', the universe normally works like this, with some exceptions like twins and Time-Turner users. Unfortunately for the universe, it's bad at counting so if someone with an already existing duplicate comes from the far future, it won't take action (that is, eject the time traveler from the universe) unless they call attention to themselves.
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[[folder:Film]]
* In the French film ''LesVisiteurs'', an object near a copy of itself (from earlier or later in the time stream) will try to merge with its past and/or future selves. ''Violently''. "Near" isn't precisely defined, but seems to vaguely obey the inverse square law.
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[[folder:Literature]]
* In ''StarWarsExpandedUniverse'', this is believed to be the case with clone madness when the clone is created if the process is accelerated, like using Spaarti cloning cylinder.
** To be more specific, it relates to the individual Force-signatures of their minds. Clones have identical Force-signatures, and this exerts pressure on their minds as they develop, even if they aren't otherwise Force-sensitive. If they are grown any faster than double-speed, their minds can't adjust to the strain, and break. So a clone army would take about ten years to grow under ideal conditions. [[spoiler:[[MagnificentBastard Grand Admiral Thrawn]] finds a way around this using ysalamiri, creatures that block out the Force as a defense mechanism against Force-sensitive predators. This allowed him to grow a clone army to adulthood in a matter of ''months''.]]
* In Creator/ConnieWillis's ''Literature/{{Blackout}}'' / ''All Clear'', a time traveler ''has'' to return before other times that he visited -- his arrival then is his "deadline". An important plot point.
** Earlier, in ''Literature/ToSayNothingOfTheDog'', this was a minor point. One character could be sent back a few days because during those days, they were unable to pick him up from his time travel.
*** Perhaps more accurately, they were unable to pick him up from his time travel because, unbeknownst to them, there was already a version of him running around elsewhere that had been sent to that time from a few days later.
** As that universe is a StableTimeLoop where paradoxes are stopped by the universe disallowing the trip from happening at all, a more interesting way of looking at ''Literature/ToSayNothingOfTheDog'' is that retrieving him from the past would have killed his near-future self, which the police would have identified as him, cause a paradox. So it refused to let his past self through.
* In DeanKoontz's Lightning, the inventors of time traveling discover that the universe has a built-in anti-paradox mechanism, where you simply get bounced back from the time-gate if you are attempting to travel to a time where you're already present (or even might be - one character tries to correct a mistake by traveling to a time a couple of minutes before he last showed up, and the universe doesn't let him).
* In the ''Literature/TimeScout'' series, you can't travel to a time where one of you already exists because you'll wink out of existence on arrival.
* In the series ''DragonridersOfPern'', going back in time to a period where you were still alive causes both versions of yourself to experience physical and mental distress. It gets worse the two selves are near each other. In the first novel, for example, F'Nor spent some five years both carrying out his normal duties in Benden Weyr, and setting up a new Weyr in the Southern Continent. They were generally alright when on opposite ends of the planet, but whenever Future!F'Nor came to Benden Weyr to tell F'lar something, the proximity caused great distress.
* In PiersAnthony's Literature/IncarnationsOfImmortality, the rule is, instead, that only three of a person is allowed to exist simultaneously. This mostly comes up in "''Bearing an Hourglass''," as the current Chronos is living concurrently with himself, but backwards, which means he is only allowed to travel to a particular point in his life once and once only.
* In ''{{Animorphs}}'', Ax states that, if you go back in time through a Sario Rip and come up to the exact same time as the moment you went back in the first place, you will be annihilated. However, since a rip can knock you back to while you were still alive, that means that there ''are'' two copies of one person at a time, and the annihilation occurs in place of a stable time loop resulting in only one copy, meaning that the person would go from two to none, instead of one.
** Turns out this is averted by another side effect of Sario Rips: [[spoiler:if future!you dies while past!you is still alive, past!you gains access to future!you's consciousness. When future!Jake dies, he snaps to his past self and simply elects not to take the mission that caused the Sario Rip in the first place.]]
* In the CallahansCrosstimeSaloon series, one of the hard and fast rules of time travel is that a particular person can only exist in one place in one particular instant in time, which means that if a time traveler needs to be somewhere else at that particular moment, he needs to either go to that somewhere else, or jump to another place in time to allow the younger or older self where they need to be. Of course, for some time travelers [[spoiler:like Mike Callahan or Lady Sally]] time travel is instantaneous, so you could excuse yourself to a back room to take care of something, disappear, and reappear after perhaps a minute or less, with no one any the wiser you were gone.
* A non-time-travel example in Creator/SergeyLukyanenko's ''Literature/{{Spectrum}}''. A bartender the protagonist meets on one planet reveals that he was one of the earlier humans to use the Keymasters' [[PortalNetwork Gates]]. A malfunction created multiple copies of him on many planets and continues to periodically create them. According to the Keymasters, the universe does not tolerate multiple copies of the same person and will try to kill all duplicates off through a series of unrelated accidents or unfortunate events (e.g. a docile animal suddenly goes berserk in his presence). The Keymasters warn the original never to use the Gates again until all the duplicates are gone, as he is not likely to make it to his destination. The main StoryArc involves a girl who ends up accidentally duplicating herself with the Gates. By the same token, each time the protagonist manages to find a copy of her, she ends up dying, except this time there doesn't appear to be an original version.
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[[folder:Live Action TV]]
* In ''Series/SevenDays'' when Parker goes back in time, the Parker from the time he goes back to disappears from existence (as does the time machine itself and anything else inside it).
** This causes a problem once, as Parker taking back something causes it to impossibly disappear out of a locked-and-handcuffed-to-a-person briefcase of the villain, cluing him in that something very very strange was going on.
** There was also an episode where Parker spilling tea on a console resulted in the Sphere jumping prematurely (i.e. before he got in). So when the alarm sounds seven days ago, the others are ''very'' surprised to see ''their'' versions of Parker and Donovan (the backup chrononaut) still around. All they have to go on are the contents of Parker's bag from the future, including a key. Luckily, Dr. Mentnor knows a genuine psychic.
* Not time-travel related, but early in ''Series/StargateSG1'', when two of the same character from alternate realities met up with one another, there would be negative "feedback" that would kill them if they stayed in the same reality too long.
** Of course, conveniently, when the alternate Sam, Jack, and Teal'c arrive to Ancient Egypt in "Moebius", the original Daniel explains that their counterparts were killed by the Goa'uld. When he asks about his version, they reveals that Teal'c killed him after ''that'' Daniel was implanted with a symbiote.
*** Another convenient example in "The Road Not Taken" has Sam accidentally end up in a parallel world while experimenting with phase-shifting technology behind a force field, while her double from that universe was doing something similar. The accident that threw "our" Sam into the other universe also killed her double.
** This is completely ignored in later episodes featuring multiple versions of the same characters. Namely, an episode ''dozens'' of SG-1s are showing up and stay there for several days with no side effects.
*** That episode gets a Handwave by saying that "entropic cascade failure" doesn't occur when the realities are close enough to each other.
* In ''Series/MirrorMirror'', things that exist at both times are prevented from crossing the mirror: If you try to take one with you, you'll be shocked instead.
* ''TheAdventuresOfBriscoCountyJr''' zig-zags this, albeit probably unintentionally. In one epsisode Brisco goes back in time and gets an Orb from his past self, telling himself that he needs it in the future. In a later episode Brisco goes back in time again to effect a BigDamnHeroes moment that went badly the first time around, but this time his past self disappears from the continuum when he does so.
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[[folder:Tabletop [=RPGs=]]]
* ''DungeonsAndDragons'' 1st and 2nd edition. When a clone was created with the '''Clone''' spell, both the original and the clone knew of the other's existence and each would try to kill the other. If they couldn't, within a week either one would go insane and kill itself (90% likely to be the clone) or, 2% of the time, both would kill themselves.
** In ''ForgottenRealms'' some mages found a way around this -- Zunroun cooperated with a ''dozen'' of his clones for some time. Preventing this by keeping all clones cold until the original dies also worked -- Manshoon's Stasis Clone spell was a great secret and major plot point, making the Black Network's leader a RecurringBoss while keeping him on a disproportionately low level due to setbacks and memory losses every time [[SuicidalOverconfidence he bit off too much to chew and got killed]].
** ''The Gates Of Hell'', a fan written ''DungeonsAndDragons'' book, features a devil who can travel in time. If he tries coming too close to his alternate self, he is pushed back and stunned.
** The ''Chronomancy'' book in 2nd edition enforced this on a planar level. A time traveler (a chronomancer and anyone they take along) could not exist in a timeframe where they were elsewhere on the same plane. The closer they got to attempting, more temporal rifts would open, causing [[ClockRoaches various temporal monsters]] to attack the travelers. If they somehow outlasted everything until the point where they would have been, a rip in the space-time continuum automatically opens up and shunts the travelers to the next time period in which they would be unique.
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[[folder:Video Games]]
* In ''InfiniteSpace'', a rogue Zenito general repeatedly used mind transfer to escape death when you killed him. [[spoiler:Franny stopped this by altering the transfer to copy the mind to all the clones, causing him to go crazy and die, because there can't be more than 1 copy of a person at a time.]]
* When the main characters of the ''VideoGame/{{WarCraft}}'' [[Franchise/WarcraftExpandedUniverse Expanded Universe]] ''WarOfTheAncients'' Trilogy are transported to the past, Krasus, the only one who existed then, finds himself considerably weaker and unable to transform into his true dragon form of Korialstrasz. We find Krasus past self is experiencing the same problems, and it's attributed to the fact that, since they're one and the same, they're sharing the same life force. They also find themselves much stronger if they are together. Eventually they're able to lessen the problem by swapping one of their scales and using magic to bind them to their skin (It hurts like hell, but Krasus is able to transform his arms just long enough to rip off one of his scales).
* ''VideoGame/CastlevaniaJudgment'' is a borderline example. While Aeon is able to [[MirrorMatch face himself in battle]], he specifically states that it will cause damage to the timeline. Since he has the power to fix it, however, there isn't much of an issue.
* In the [[MirrorUniverse Mirror Realm]] of ''VideoGame/AdventureQuestWorlds'''s 2nd Birthday Event, the hero that you play is chosen to save the Mirror Realm because he or she is the only person in the entire multiverse who does not have an alternate in the Mirror Realm -- if anyone else in either world were to go to the other world, they'd have to switch places with their mirror counterpart. Paul and Storm do not have mirror counterparts either because they "grew up in a small suburb just outside of the multiverse." [[{{MST3K Mantra}} Just roll with it]].
* In ''VideoGame/TheLegendOfZeldaMajorasMask'', no matter how many times you go back in time, there is only ever one Link (and Tatl) in Termina. Most likely, [[AWizardDidIt this is the Ocarina of Time at work.]]
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[[folder:Web Comics]]
* ''{{Homestuck}}'': According to Aradia, a future self who travels back from a [[BadFuture doomed timeline]] is doomed to be destroyed by the universe sooner or later.
** This is only a partial example, as people can travel through time without problems as long as they stay in the same universe.
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[[folder:Western Animation]]
* In ''WesternAnimation/JusticeLeague'', Vandal Savage's time machine cannot send a person back to when that person existed. Since he's an [[{{Immortality}} immortal]] who's lived through around 99% of human history this means he can't actually use it himself.
* ''{{Futurama}}: Bender's Big Score'' had this in effect. When ever a duplicate was created through time travel the universe would kill them off to prevent further confusion. However, at the end of the movie, several hundred time-traveling Benders appear simultaneously; this proves too much for the universe to handle, and a hole in space-time rips open.
* In ''WesternAnimation/OggyAndTheCockroaches'', characters traveling to the past "merge" with their past selves upon meeting to avoid this problem.
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