History Main / OnlyOneMeAllowedRightNow

* ''{{WesternAnimation/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 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.

* ''Series/TheManInTheHighCastle''': One of the metaphysical laws of dimension-traveling between alternate universes is that a person cannot co-exist with another version of themselves in the same reality. Therefore, one can only travel to a universe where [[DeadAlternateCounterpart their counterpart is dead]].

* The most important time-travel rule on ''Series/Timeless''. It is impossible to revisit any time-period where another version of the time-traveler exists. This means that time-travelers cannot travel to any point within their own lifetime, or revisit any time-period they have previously traveled to. Its mentioned in the pilot that one attempt to do so resulted in the time-traveler returning mutilated.

* ''Franchise/StarWarsLegends': 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, due 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 ''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''.]]

* 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).

** The ''Warlords of Draenor'' expansion makes a point of killing off the alternate timeline's Velen, for no real reason except that he's one of the few characters to be alive in both timelines who's presence in the story could not logically be ignored.

** Merorem from ''TabletopGame/TheGatesOfHell'' can travel in time. If he tries coming too close to his alternate self, he is pushed back and stunned.

** Merorem ''TabletopGame/TheGatesOfHell'', can travel in time. If he tries coming too close to his alternate self, he is pushed back and stunned.

** ''The Gates Of Hell'', a fan written ''TabletopGame/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.

* This applied to the Franchise/DCUniverse before the first ComicBook/{{Crisis|on Infinite Earths}}. 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.

** Some early [[UsefulNotes/TheSilverAgeOfComicBooks Silver Age]] 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.

* 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 (well, actually Madelyn Pryor, but [[CloningBlues close enough]]) and Scott Summers had a son... she never had a brother.

* In the ''VideoGame/SuperRobotWarsZ'' plotline, this trope is the reason why [[Manga/MazingerZ the original Kouji Kabuto]] and the [[Anime/GetterRobo classic Getter Robo team]] can't team up with their ''Anime/ShinMazinger'' and ''Getter Robo Armageddon'' counterparts.

* In Creator/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.

* This applied to the DCUniverse before the first ComicBook/{{Crisis|on Infinite Earths}}. 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.

* ''{{Webcomic/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.

* In ''Webcomic/LsEmpire'' you can't time travel to a point where you were already alive (you will be sent back to your time of origin upon birth). Note that this only applies to mechanical time travel, magical time travel doesn't have any set rules.

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.