History Main / OnlyOneMeAllowedRightNow

* In ''LightNovel/EightySixEightySix'', Shepherds, who have full human personalities, seem to be unable to withstand having more than one of it being in existence, unlike Black Sheep which can exist as multiple copies since they don't have personalities. This becomes a problem for the Legion if a Shepherd is getting destroyed, since they can't have backups, they don't seem to transfer the personalities until it's way under emergency, and trying to transfer full consciousness in that condition and in short amount of time is impossible. Thus, even if the personality is transferred over, it's heavily damaged.

* ''WesternAnimation/GravityFalls'': "[[Recap/GravityFallsS1E9TheTimeTravelersPig The Time Traveler's Pig]]" also had this in effect. When Dipper and Mabel steal a time traveling device lost by a time traveler from the future, they use it to travel back in time to the start of Stan's fair, all the while never running into their past selves from eariler since there aren't any instances of time travel duplicate at all.

** 3rd Edition solved the problem by making the clone inert, even rotting unless preserved, until the original died and then his soul would instantly transfer to the clone.

** The ''Timetricks'' supplement, which added more freeform time traveling to the game, introduced a device that would let you bypass that law for a short period of time ... if it worked.

* In ''LightNovel/EightySixEightySix'', [[spoiler:Shepherds, who have full human personalities, seem to be unable to withstand having more than one of it being in existence, unlike Black Sheep which can exist as multiple copies since they don't have personalities. This becomes a problem for the Legion if a Shepherd is getting destroyed, since they can't have backups, they don't seem to transfer the personalities until it's way under emergency, and trying to transfer full consciousness in that condition and in short amount of time is impossible. Thus, even if the personality is transferred over, it's heavily damaged.]]

* ''{{WesternAnimation/Futurama}}: Bender's Big Score'' had this in effect. Whenever 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/SamuraiJack'', a person cannot travel to any point in time that you have existed in. This is why Aku has to prevent Jack from travelling back; If Jack gets through, Aku would be unable to follow him even with his own power, and he's existed for a few millennia since he threw Jack forward. [[spoiler:This is best demonstrated when Jack could only return at best seconds ''after'' the moment Aku flung him out, and not at any point in his own lifetime before]].

* This happened to Comicbook/{{Jubilee}} in an issue of ''Comicbook/{{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.

* In ''LightNovel/EightySixEightySix'', [[spoiler:Shepherds, who have full human personalities, seems to be unable to withstand having more than one of it being in existence, unlike Black Sheeps which, since they don't have personalities, can exist in multiple. This becomes a problem for the Legion if a Shepherd is getting destroyed, since they can't have backups, they don't seem to transfer the personalities until it's way under emergency, and trying to transfer full consciousness in that condition and in short amount of time is impossible. Thus, even if the personality is transferred over, it's heavily damaged.]]

* In ''LightNovel/EightySix'', [[spoiler:Shepherds, who have full human personalities, seems to be unable to withstand having more than one of it being in existence, unlike Black Sheeps which, since they don't have personalities, can exist in multiple. This becomes a problem for the Legion if a Shepherd is getting destroyed, since they can't have backups, they don't seem to transfer the personalities until it's way under emergency, and trying to transfer full consciousness in that condition and in short amount of time is impossible. Thus, even if the personality is transferred over, it's heavily damaged.]]

* 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.

* This applied to the Franchise/DCUniverse before the ''ComicBook/CrisisOnInfiniteEarths''. 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.

* A {{Downplayed}} example in the ''Literature/TheWarOfTheAncients'' novels of the Franchise/WarcraftExpandedUniverse had the mage Krasus (a.k.a. the dragon Korialstrasz) go back in time with several of his companions. Krasus found himself immediately having health problems as well as being unable to transform back into a dragon, and his counterpart had the same health problems though he was a dragon for the entire episode. When they were in physical contact, the problem was relieved a bit. Krasus suspected that the laws of his universe held to this trope, only allowed one of him at a time, and being back in time compromised the "oneness" of both of his selves.

* When the main characters of the ''VideoGame/{{WarCraft}}'' [[Franchise/WarcraftExpandedUniverse Expanded Universe]] ''Literature/TheWarOfTheAncients'' 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).

* This is central to one character in ''VisualNovel/FateStayNight'', but even saying which is a huge spoiler. [[spoiler:Archer is actually counter-guardian EMIYA, as in Emiya Shirou, as in the idealistic protagonist he shares a mutual hared with. Archer is Shirou's future self from a possible future, and merely existing at the same point in the timeline causes the two to bleed skills and thoughts into each other (well, mostly one way, since Archer sees Shirou as an unskilled idiot). Archer's suicidal actually hoping this disruption - or failing that, [[TemporalParadox killing his younger self personally]] - will finally put a permanent end to his existence.]]

* In the many time-traveling episodes of ''Series/OddSquad'', a rule is stated that if your other self sees you, it will cause a "Timetastrophe," which causes time to fold in on itself. The Timetastophe has three phases: First, there are the timequakes. Then, the opening of time holes let in a 1920s businessman, a viking, and a gogo dancer. Lastly, the worst of all, the time sheep appear.

* In the many time-traveling episodes of ''Series/OddSquad'', a rule is stated that if your other self sees you, it will cause a "Timetastrophe," which causes time to fold in on itself. This causes timequakes, the opening of time holes letting a1920s businessman, a viking, and a gogo dancer come through, and the appearance of the Time Sheep.

* A central point of the time travel mechanics in ''VideoGame/RadiantHistoria'' is that Stocke can only be in one place at one time. When he travels back to a previous point in the timeline, he'll also reappear where he was at that point. While this isn't usually a big issue in the story, it does create problems when a rival time traveler attempts to [[CuttingTheKnot cut the knot]] on Stocke's efforts by attacking a key figure before Stocke met them. [[spoiler:Of course Stocke happened to be passing by on another mission and may have bumped into said character if you went to the right area. When he appears to intercept the assassination, said rival is flummoxed for a moment and [[LampshadeHanging laments out loud]] that ''of course'' Stocke would have just happened to have been this close at this point in time.]]

** Some early [[UsefulNotes/TheSilverAgeOfComicBooks Silver Age]] Franchise/{{Superman}} stories (& one [[Franchise/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.

* 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 Comicbook/RachelSummers (daughter of Comicbook/JeanGrey and [[Comicbook/{{Cyclops}} 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 ''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.]]

* In ''LightNovel/EightySix'', [[spoiler:The Legion Shepherds, who have full human personalities, seems to be unable to withstand having more than one of it being in existence, unlike Black Sheeps which, since they don't have personalities, can exist in multiple. This becomes a problem for the Legion if a Shepherd is getting destroyed, since they can't have backups, they can only be transferred under emergency, and trying to transfer full consciousness in that condition and in such amount of time is impossible. Thus, even if the personality is transferred over, it's heavily damaged.]]

* In ''[[LightNovel/EightySix]]'', [[spoiler:The Legion Shepherds, who have full human personalities, seems to be unable to withstand having more than one of it being in existence, unlike Black Sheeps which, since they don't have personalities, can exist in multiple. This becomes a problem for the Legion if a Shepherd is getting destroyed, since they can't have backups, they can only be transferred under emergency, and trying to transfer full consciousness in that condition and in such amount of time is impossible. Thus, even if the personality is transferred over, it's heavily damaged.]]