"Alright, this next test may involve trace amounts of Time Travel. So, word of advice: if you meet yourself on the testing track, don't make eye contact. Lab boys tell me that'll wipe out time. Entirely. Forward and backward. So do both of yourselves a favor and just let that handsome devil go about his business."
For some reason, encountering yourself—whether as a time-traveler or in Another Dimension—is frequently Very Bad; either it's unhealthy for you in particular, or it creates a Temporal Paradox. While this is frequently incredibly useful as a plot device, it doesn't make sense; if there are negative effects to existing simultaneously with yourself, why should they depend on proximity? And why would it require that you as a unit be next to yourself, rather than, say, the atoms in your body (of which there may be few left if you meet yourself as a baby)?
This may result from attempts to apply causality to time travel: you can't meet a past version of yourself without having memory of it and the future version of yourself cannot be surprised to see past you, having had to be there to see it to make it possible. However, this can be easily avoided if you fail to recognize yourself. As a result it's generally correlated with time travel of the Timey-Wimey Ball variety, and negatively correlated with Stable Time Loops (though there are exceptions). Compare Future Me Scares Me; contrast Screw Yourself.
Note that this trope is now so well known that movies which use Time travel often only throw out a one-liner warning: "Whatever you do, don't meet up with yourself!" It's now becoming a trope in and of itself to make the reason for not meeting up not a dire end-of-the-world reason, but merely for convenience of the Master Plan. These often result in My Future Self and Me.
A closely related trope is Only One Me Allowed Right Now.
Future Asahina conceals herself from her past self almost every single time, with the explanation that her past self didn't see her future self. The one exception is when Kyon, Asahina (small) and Nagato go back to December 18th to save the world; Asahina (small) sees Asahina (big), but doesn't recognize her, and doesn't pay her much attention. There are slightly more important happenings going on, like, y'know, Kyon bleeding to death in the middle of the street..
In The Disappearance of Haruhi Suzumiya we see Present Kyon hide so as not to reveal himself to Past Kyon from "Bamboo Leaf Rhapsody," who he remembers didn't see him; but towards the end, when Kyon gets stabbed, his future self arrives and tells him some words before he passes out. After he wakes up, Kyon recognizes that the voice he heard was his future self, and that when he travels back in time to that day again, he must tell those words to his past self.
In the manga version of Sailor Moon, during the Time Travel to the thirtieth century, the titular character feels faint in the presence of her future self, Neo-Queen Serenity, and her body becomes transparent; her Silver Crystal also loses its power under the influence of its future counterpart. At the end of this story arc, Neo-Queen Serenity tries to resist the temptation of talking to her past self, since it may result in the history being changed... fails, and goes to meet Sailor Moon anyway. The story also involved Prince Demand attempting to bring the world to an end by bringing together the two Silver Crystals. Interestingly, in Stars it appears that the selves can meet, if one of them is (supposedly) from an alternate distant future and in disguise. Possibly.
Played straight in Kimagure Orange Road. Touching your past self will cause the future version to dissolve. Never really explained exactly what happens to the future self but it gets shelved under "a bad idea to try".
Played with quite a bit in Mahou Sensei Negima! during the Mahorafest arc. The aim is to not let the selves meet, but often the later selves run into the earlier selves, and at times, a character runs into one version of a character almost immediately after leaving another version of the character, causing confusion—especially when it happens with one of the girls not in on the Masquerade. Also, at one point, we distinctly see two Chamo-kuns meeting up and high-fiving. And then switching places! How that's supposed to work is mind-boggling
Natsu no Arashi! firmly enforces this. People start to fade when they get close to their past selves, and if you meet yourself, you'll vanish entirely. Time doesn't like people having two of themselves in one place.
In Kurokami, every person has two dopplegangers. When they meet two of the three wither away, but the third will thrive at the cost of the others.
In Katekyo Hitman Reborn! the past and future versions of the Vongola are not able to coexist in the same timeline.
Actually, at the end of the TYL! arc, we find out that the TYL! Vongola guardians are being kept in some weird capsule thingy but do see/talk to their past selves. Although they were admittedly in what could have been a separate pocket dimension created by the current Big Bad 's men.
Seikimatsu Occult Gakuin goes for the worst possible variation. Fumiaki meeting his past self overloads the timeline and causes the apocalypse. Though he does Screw Destiny and prevents the thing he caused.
In the JoJo's Bizarre Adventure story arc Steel Ball Run, this trope is applied to versions of people and objects from parallel universes. In "Dirty Deeds Done Dirt Cheap", the Stand of the Big BadFunny Valentine, specifically protects him from the effects of this, but also allows him to use it against his enemies.
Even after his defeat, the final enemy ( an alternate Di(eg)o Brando) still meets his end through this rule courtesy of Lucy Steel and the decapitated head of the main universe Diego.
This is actually used in the All-Star Battle video game as the character's strongest attack - he summons a version of his opponent from another universe, and then throws the two of them into each other, causing both to detonate.
An old Silver Age story involves Wonder Girl (Wonder Woman as a girl, before she became a separate character) attempting to meet her adult self but failing; this is the reason given for the failure. Later, it's why there needs to be a Hand Wave ("Amazon magic") for why Wonder Girl and Wonder Tot (Wonder Woman as a toddler) are able to team up with the adult Wonder Woman.
Superman:(looking away) Right now Supergirl is... in the past.
In most Silver-age Superman stories, if the Man of Steel travels to a time, past or future, where he exists, that other version gets tossed back to the "empty slot" he started from. This normally occurs in Superboy stories, so that Superman ends up wandering the Smallville of his youth.
In the Final Crisis: Legion of 3 Worlds miniseries, Superboy-Prime punches his future self, causing both selves to dematerialize and disappear.
In Final Crisis: Superman Beyond it was claimed that Superman and his Mirror Universe counterpart, Ultraman, can't touch each other without causing an explosion that would destroy them both. Except in Limbo, where the rules are different. (Never mind that they've fought each other lots of times and that's never happened before.)
Also, the first time one version of Supergirl met Power Girl, shaking her hand caused Power Girl to suddenly go berserk and attack everyone. This was attributed to them being alternate universe counterparts and it making reality glitch out for a moment or something, but it hasn't happened since.
In the New 52 Supergirl issue #19, Supergirl and Power Girl meet up again with very different results, providing both a discussion/lampshading and subversion of this trope. It turns out Power Girl knew about the main DC Earth's version of Supergirl but didn't dare because she didn't want "the universe to explode if we touch." (What actually happens when they do touch is that they exchange memories.)
In Timecop, you can meet yourself and even talk to yourself, as long as you do not touch, because "the same matter can't occupy the same space at the same time". When the Big Bad does (with the hero's "help"), both selves melt and are erased from time.
Major Victory of the Marvel Universe, after he travels back to the present, has to be careful about this with his shield, since it's Captain America's from the future; it's established that if the present and future shields ever make contact, Bad Things will happen. Oddly, this is not a problem when he meets his younger self.
One Wolverine comic involves Jubilee going back in time. Her past self temporarily disappears.
In one anniversary edition of Spider-Man involving Doctor Strange, Spidey was successfully able to see his past self (right before being bitten by the spider) and listen to advice from his future self on what seemed to be the end of his career (right before his future self is gunned down by the police for some unknown crime). Of course, seeing as the reason he was seeing this was because the fabric of reality was already in jeopardy because of Dormammu's plot to bring about the apocalypse, it's likely these two encounters couldn't have done any further damage.
Winx Club: The comic book version has a story where the Winx girls travel to the future. The scientist who invented the Time Machine discourages them from meeting their future selves because of this trope.
Around the time Titanic came out, a lot of Doctor Who fanfics got written with the Doctor and co. visiting the doomed ship. One fanfic lampshades this by having the TARDIS appear on the Titanic only for the Doctor to moan, "Oh not again!" His companions look around and notice that a large percentage of the passengers are different versions of the Doctor and his various companions. "The TARDIS just seems to like the North Atlantic," he sighs.
There was a My Little Pony: Friendship Is Magic fanfic which invelved Rainbow Dash travelling in time to set herself and Applejack up. Eventually, because of this, she, Applejack, and Twilight Sparkle are all jumping through time trying to make sure the right versions of themselves meet at the right time. It ends up with several dozen of each pony getting very confused. And then Scootaloo gets caught up too...
Another fanfic, On a Cross and Arrow, instead had dimensional travel replacing time travel. Twilight ended up sending the Mane Six into an alternate dimension where everyone is gender-swapped versions of themselves. She feared that if they met their alternate selves, they would explode or something. Thankfully, this isn't the case in the slightest.
Another variation shows up in the Pony POV Series, during the Dark World arc when Twilight meets Minty Pie. The variation is that both of them are reincarnations of G3 Minty note Twilight is Minty's "Light" (her soul) while Minty Pie is her "Shadow" (her physical appearance). Due to this unusual variation, they're able to interact fine, without any problems, but when they make physical contact they start to fuse together which pretty much Mind Rapes them both. Fortunately, they break apart in time to avoid anything permanent, and while shaken, they're fine afterwards.
Back to the Future, as noted on the quotes page. Probably the Trope Codifier in film, despite the trope being less a hard and fast rule and more of a general guideline.
In the movies, nothing worse than fainting happens, and both Doc and Biff seem able to avoid even that by averting their eyes from themselves, or maybe just by being prepared for it. It seems that young Biff in the '50s had no basis to suspect that the "crazy old codger" was himself. It's also entirely possible that the "destroy the universe by temporal paradox" hypothesis was just that - a hypothesis, and there never was any real danger at all.
The two Bobs explain that Doc's concern is that the recognition of a past character meeting his or her future self could lead to an event that causes the paradox; for example, while the two Jennifers simply passed out when meeting each other, the producers explain that had "young" Jennifer fallen, hit her head and sustained a fatal injury she would not have had a future self to trigger the incident, resulting in a paradox.
Likewise, in the 2011 video game, the beginning of Episode 2 requires Marty to retrace his steps and keep his grandfather from getting killed while trying not to run into himself from a few hours ago (during the events of Episode 1). To make things even more confusing, the end of the final episode of the game has Marty arguing with Marty and Marty over which timeline is correct, while a confused Marty looks on. And yet Doc still says, while all of these Martys are having their little discussion, that them meeting each other could destroy the space-time continuum. Still seems fine at the moment.
In LJN's Back To The Future II & III game for the NES, though, as you leave behind temporal clones of yourself every time you leave and then revisit one of the three time periods (1955, 1985, and 2015) in the Part II section, collision with your temporal copies causes you to lose a life.
The ride at Universal Studios ends with a frantic order from Doc to leave the DeLorean before you encounter your past self coming in.
In Happy Accidents, Sam explains that it is impossible for time travelers to travel back within their own lifetimes; the only time travel possible is movement far into the past.
In the film Southland Tales, a huge part of the plot hinges on this twist, revealed late in the film:
Two time-travel-created copies of the same person shake hands with each other, setting off the end of the universe.
Boxer Santaros avoids this as someone has already taken the care to murder his double. Or perhaps the original.
Subverted in Primer: when the protagonists start time traveling, they take elaborate precautions to avoid meeting themselves, but it becomes progressively more clear that (aside from creating permanent duplicates of themselves) these precautions are totally unnecessary. By the end of the movie, Aaron has drugged his past self's breakfast and stowed him in the attic, and is then promptly attacked by yet another, future, version of himself.
Played with in Star Trek: Spock Prime insists that Kirk cannot tell the younger Spock about his existence, with the heavy implication that some kind of universe-ending unpleasantness would ensue if he did. At the end, Spock Prime then seeks out and introduces himself to himself, and all-but-admits that he lied to Kirk, or at least deliberately left the universe-ending implication open. This was so that young Spock would eventually learn to develop the same bond that existed between Spock Prime and the Kirk of his reality.
Super Capers seems to be fine with the two meeting each other, but if they physically touch each other... that's a different story.
Déjà Vu took care to avoid potential paradoxes like this in an unwritten way by ensuring that the Denzel who traveled back in time died without ever meeting his younger self.
In Lost Highway, Fred Madison buzzes himself on the intercom to tell himself, "Dick Laurent is dead." A David Lynch film, of course, this is open to interpretation.
Godzilla vs. King Ghidorah involves a Time Travel plot to remove the original dinosaur that became Godzilla from an island occupied by Japanese Troops in WWII. When one character asks why they couldn't bring one of the Japanese WWII veterans they knew with them, since they'd know where the dinosaur would be, the Futurians who had the time machine explain that if the same person was in the same point in time twice, it would cause a paradox that would cause one or both of the person in question to be wiped from existence.
Played straight to gruesome effect in Time Cop. Physical contact with your other self leads to... well, it's not pleasant. As mentioned in the comic book entry, this is specifically due to the same matter occupying the same space; as long as they didn't touch, meeting was fine.
Note that both selves will feel the effects. If you suddenly feel dizzy and weak for no reason, it might mean that future-you is in the vicinity.
In Moreta: Dragonlady of Pern, to stop a plague, Moreta and her dragon repeatedly loop back over the same time period. Though her appearances are in many different locations, the repeated trips prove lethal.
The most recent books imply that this problem is exclusive to dragonriders, and is the result of their telepathic bond with their dragon inadvertently becoming duplicated as a result of the time travel.
This reaction is the reason Lessa survived Fax's attack on Ruatha Hold. When adult Lessa unwittingly traveled to the morning of the attack, child Lessa was awoken by the feeling of unease brought about by her adult self's presence and instinctively hid in the watch-wher's kennel.
The Star Wars Expanded Universe novel The Last Command by Timothy Zahn has a variant of this trope, involving clones instead of time travel. Clones grown near others set up "resonance effects" in the Force, which can drive them insane, and when Luke encounters his clone, he finds the experience incredibly off-putting: there's a buzzing hum in the Force that makes it hard to concentrate or think, making it difficult to fight the clone. This only seems to affect clones whose growth has been accelerated to an extreme degree — having them grow up in under a year, for instance — and Force-sensitive clones, since the clone army in the prequels (who take about 10 years to grow up) doesn't have any problems with this. Thrawn figures out a way around it in the same novel, using lizards that can push away the Force to protect against the clone madness and dramatically decrease the time to grow them even further.
One of the most important rules of Time Travel in Harry Potter? Don't be seen by yourself. You could panic and kill your past or future self. Notable for not really being a result of time travel so much as just being a duplicate. Though not spelled out explicitly in the books, this could have something to do with the existence of Polyjuice Potion — in most non-time-travel related situations where you notice a duplicate of yourself, it means someone's up to no good. As Harry's own experience with Time Travel, which was the only one the readers were able to follow, turned out to be a Stable Time Loop, the killing-your-past-self thing is unlikely to ever happen.
The Time Scout series, which already presents time travel as potentially dangerous if you don't get the small details right, also avoids the meet-yourself problem by making it clear that you must never go back to a time when you already exist, either by having been born by then, or having previously visited via time travel; otherwise, since you can't be in two places at once, the "current" you who went back in time would simply wink out of existence the instant you arrived, and that's the end of you.
In Connie Willis's time-travel books, you simply can't go back to a time you've already visited. If you try it—or if your presence, for any reason, would cause a paradox of some sort—either the "net" (via which one time-travels) simply won't open, or it will deposit you slightly awry (in time and/or space) of your intended destination. Cosmic Censorship may be at work here: if you cause a paradox, then the next version of you is sent to a slightly different point (or the machine fails), repeat until there's no paradox.
Subverted in The Hitchhiker's Guide to the Galaxy novel Life, the Universe and Everything; Ford Prefect and Arthur Dent arrive on Earth shortly before its destruction. Ford (who had, in the previous novel, explained to Arthur that history cannot be changed because it all fits together like a jigsaw), warns Arthur against phoning to warn himself. Not because it'll do anything to the timestream, but just because it won't work. It had already been noted in The Restaurant at the End of the Universe that no matter how many times you visit the restaurant in the title, which you would always do in the universe's last half-hour or so, you are guaranteed to never run into yourself "because of the embarrassment this usually causes," despite this being impossible. How the people responsible for the restaurant's operation pull this off is not explained, but it is lampshaded magnificently along with other things about Milliways by the Guide's repeated use of the phrase "This is, of course, impossible", and the restaurant's advertising slogan: "If you've done six impossible things today already, why not round it off with breakfast at Milliways, the Restaurant at the End of the Universe?".
Diana Wynne Jones's novels often involve alternate universes and occasionally also time travel, which means they touch on this from time to time:
A Sudden Wild Magic has an example similar to the Stargate one below — travel into alternate universes is possible, but causes instant death for anyone with a counterpart in that universe.
In Charmed Life, the character who manages to travel into an alternate universe does so via a method which cyclically displaces all her alternate selves, so every universe that had a version of her continues to do so. In fact, the main qualification for Chrestomanci, the enchanter who keeps all the other magic users in line, is that he have nine lives. This makes it possible for him to go to other universes comparatively easily: he has the extra lives because the alternate selves who would otherwise have them don't exist.
But in A Tale of Time City, Vivian, Jonathan and Sam manage to be on the same railway platform in three different incarnations at the same moment and nothing happens - although they're careful not to be seen, it's just in case they change history. More than they already have, anyway.
In a Sonic the Hedgehog novelisation Sonic the Hedgehog in the Fourth Dimension, older and younger selves could co-exist, but not touch: if they did, they melted together somewhat painfully to become one self. Sonic did this ("Now I'm twice as cool"). He also did this with the evil anti-Sonic, Cinos, causing some alarm as to whether good or evil would win out in the resulting Sonic. Good won. Imaginary creatures threatening to rewrite history kept people from asking too many questions. Still, at the end, it was two positive Sonics plus one negative Cinos, which should equal one positive Sonic.
In Johnny and the Bomb by Terry Pratchett, the time-trolley sends Johnny forward in time just before his past self sees him, apparently to avoid paradox. It's still enough to shake the time travelling Johnny: "I saw the back of my own head! No-one's done that since the Spanish Inquisition!"
In the Warcraft time-traveling trilogy War Of The Ancients, the character Krasus (a dragon shapeshifted into an elf) is unable to transform into his dragon form because he already exists, as his younger counterpart Korialstraz. Likewise, Korialstraz is unable to transform into anything else. The two are weaker the farther apart they are, but become stronger when they're close. The two work around this problem somewhat by giving each other one of their scales.
Subverted in book six of the Artemis Fowl series, appropriately titled "The Time Paradox". The premise is that in order to save his mother, Artemis must travel back in time to stop his past self from wiping out the last of the silky sifaka lemurs, whose brain fluid can cure her. Artemis meets himself, does battle with himself several times (and Butler, who's arguably the more dangerous of the two), and eventually the two Artemises collaborate to bring down Opal Koboi.
In The End of Eternity, there are time periods in regular time allocated for an Eternal to use, and naturally, you are not to use them twice. However, when the protagonist goes into regular time illegally, he doesn't keep track as well, so he almost meets himself (that is, he catches a glimpse of his past self). There don't seem to be any consequences, but he is extremely and irrationally terrified.
In The Book of the New Sun, Severian feels absolutely certain that if he accidentally met himself while time traveling, one or both of them would go mad and kill the other.
Hinted to normally be the case for the Eternal Champion in Michael Moorcock's related stories. Different incarnations do meet each other and even team up from time to time under suitably unusual circumstances, but once the emergency allowing for the encounter passes they inevitably have to part ways again shortly thereafter (sometimes downright abruptly) to avoid putting too much unspecified strain on the multiverse.
The Rifter: Averted. When Kahlil arrives in a Basawar where history was changed, the version of himself in the new time is already dead. He does have two sets of memories coexisting confusingly in his head, though. Eventually, he finds the bones of his other self and they merge into his body; at the same time, he feels like his two histories have become truly integrated into one personality.
In one of the R Is For Rocket stories, a man participating in a time-travel safari has this explained to him. The 'bump' they felt just before arrival was the time machine leaving at the end of the safari, the tour guide explains that "Nature doesn't allow that kind of thing- man meeting himself".
In Larry Maddock's "Agent From T.E.R.R.A." series about time traveling cops trying to defeat a rogue time traveling agent turned conqueror, this is explicit. Why is never explained, but at least once per book the plot hinges on one or more people being prohibited from entering a certain period of time (minutes to hours) because they were then (sic) already. One very nice feature of this series is that several times the main character will race toward his time machine, immediately phase out of the time stream, and then relax: going to bed, taking care of wounds, preparing and eating a hearty meal, etc. He reflects that he could spend literally years preparing to go back to the precise micro-second he left, so now there is no hurry at all.
This rule is on the rather extensive list of time travel restrictions in Dean Koontz's Lightning. This becomes a serious problem at the end of the book when someone travels very, very briefly to a point in the future and can't return to it, even though he really, really needs to.
Live Action TV
This has, not surprisingly, come up a few times on Doctor Who, though he has had no trouble (usually) meeting past versions of himself.
The Doctor, while visiting a fascist Mirror Universe of Earth, claims that if that universe's evil version of the Brigadier were to meet the original it would destroy both universes. The evil Brigadier wants to force the Doctor to save him from his Earth, which volcanic eruptions would destroy in a matter of minutes.
In "Father's Day", traveling to the same place twice allows Rose to create a Temporal Paradox; later in the same episode, she gives more power to that story's Clock Roachesby touching her own infant self. It's revealed that cleaning up paradoxes without such things happening is part of what the Time Lords did, but now that (for the most part) there aren't any Time Lords any more, there's no one to prevent such things from happening. This may explain why no flying killer time monkeys appeared during any serial with a title of the form "The [number] Doctors."
The reason that it was such a dangerous deal when Rose touched her infant self is because she had accidentally caused the time-sterilising monsters to show up by saving her father's life when he had originally died. The later paradox of touching her infant self is only dangerous because it lets the monsters into the church. It's implied that in normal circumstances, this would just be a paradox and nothing dangerous would necessarily come of it.
Mawdryn Undead shows that when the Brigadier met his younger self, the resulting "Blinovitch Limitation Effect" had the effect of giving the younger Brigadier a nervous breakdown, as well as enough Pure Energy to kill a half-dozen previously immortal aliens. In terms of the Brigadier, this created a neat Stable Time Loop.
Of course, the original series also has several stories where multiple incarnations of the Doctor were brought together in the same place at the same time, without any apparent ill effects to the Timey-Wimey Ball that is the Whoniverse. (Most of the time, this could be explained by the fact that the multiple incarnations aren't entirely the same person. But a couple of times two of the same incarnation have met.)
The 2007 special Time Crash blamed Peter Davison's Doctor aging 30 years on temporal weirdness resulting from being in the same room with himself. (This same explanation may be used to justify the relatively minor differences in appearance occurring in multi-Doctor stories in the original series- the biggest of which (besides The Other Darrin example of the new First Doctor) was the obviously-older Second Doctor in "The Two Doctors". But then the Doctor Who Expanded Universe explains the latter, anyway.)
Funnily enough, this flies out the window in "The Big Bang" when Amy meets her seven-year-old self ("Amelia"), and touches her a couple of times in sizing her up. Possible Fan Wanks include a) Amelia is from an alternate timeline, so they're not technically the same person (apart from anything else, she vanishes minutes later, so she obviously doesn't grow up to become Amy), and b) with the whole Universe toast, save for the Earth and the TARDIS explosion acting as a substitute Sun, there are probably no Clock Roaches, and really more pressing matters at hand than a couple of silly old paradoxes.
Lampshaded later when Amy mentions that she met herself, and Rory responds that, to be fair, the universe did blow up.
Another time, the "limitation effect" was a small spark between two of the same Sonic Screwdriver.
In Big Finish Doctor Who story "Time of the Daleks" the Eighth Doctor claims if two versions of someone meet the older version will be destroyed, as the younger version needs to become the older version. It is revealed this is the reason General Learman destroyed the Dalek Pilot. It was because of the lack of a pilot that the Daleks turned her into one and they are trapped in a Stable Time Loop.
The never-aging Captain Jack, while trapped in 1901, orders himself cryogenically frozen to avoid meeting in the intervening decades before the present, and to prevent himself from meeting his "Boom Town" self in 2006.
At the end of "Kiss Kiss Bang Bang," the whole team, having gone back to earlier that day, are told that they must avoid themselves.
It seems, though, that they're more worried about changing their own history than a Time Crash resulting from simply being in the same place, since the same person being in the same place has happened so many times safely. If you interfere where you've already interfered, though, the Clock Roaches will getcha, as seen in the aforementioned DW episode, Father's Day.
In Seven Days, whenever Parker travels back in time, his younger self, the sphere, and one trip's worth of their alien fuel source vanishes.
Generally averted in Heroes, as several characters have met past or future selves without anything happening because of it. It's occasionally discussed, though:
The Genre Savvy Time Traveler Hiro Nakamura runs into his future self, only to cry in dismay, "Aren't you afraid of disrupting the Space-Time Continuum?"
And before that, when he realizes he's accidentally ended up talking to his past self on the phone, he hangs up instantly, exclaiming "Great Scott!"
Season four had Samuel explain to Hiro the dangers of the butterfly effect when Hiro goes to change an event with major influences on events surrounding his past self.
A non-time-travel variant occurs in Stargate SG-1: anyone who travels to another dimension is subject to quantum seizures if they have a living counterpart. You can survive for short periods, and are exempt if your counterpart is dead, which saves Daniel in one episode, Kowalsky in a second, and Carter in a third. Time travel, on the other hand, doesn't seem to have this effect in any way.
This trope gets smashed into the ground with the character of Martin Summers in Ashes to Ashes. Then again, in the light of the finale, whether it actually applies is debatable.
In Crime Traveller this is one of the rules of time travel. It was never entirely clear what would happen if someone met their past self, but the implication was that it wouldn't be good. Strangely, one of the other rules was that the time travellers needed to be back in the room with the time machine at the moment they left. In theory, this means that they should see themselves leaving and break the first rule but that was never shown to happen.
NBC's Journeyman doesn't even bother to explain why you shouldn't be seen by your other self — it just assumes you already know that. Though the main character is able to have a fistfight with his past self, so it's at least OK as long as your past self doesn't get a good look at you.
Like Back to the Future, this winds up less of a cosmic rule and more of a guildeline to prevent paradoxes, since the timeline is very malleable (case in point, the time he left a digital camera in the 1980s, which subsequently retconned his own son out of existance). But the protagonist is very Genre Savvy from the get-go.
Red Dwarf does this too, at the end of series 6. The crew's future selves travel back in time to meet them as they are at that point for some repairs. Kryten admonishes the crew to not contact the future Dwarfers to avoid gaining inappropriate knowledge of the future. They watch instead, and despise what they became. The result? 'I say we fight. ... Better dead than smeg.' From the snivelling, whining coward (according to everyone, including himself) Rimmer. Causes a paradox anyway, but that's what they wanted at that point.
In the outtake "Orchid Orientation Film" from the LOST season 3 DVDs, a time travel experiment involving bunnies apparently goes wrong. The scientist shields one of two identical bunnies from the other and shouts, "Don't let them near each other!" The series proper averts the trope, though.
The Star Trek episode "The Alternative Factor" involves two identical men named Lazarus. One is from our universe and insane. The other is from an anti-matter universe. Apparently, if both meet in the same universe, it destroys both of them. No explanation was given why the anti-matter Lazarus didn't blow up upon contact with any matter from our universe. The episode ends in a Fridge Logic moment where Kirk traps the two between universes, so they can fight it out for eternity. Apparently no one thought of killing or imprisoning the insane one.
Played with in the So Weird episode "Pen Pal". Annie is able to meet herself from a parallel universe without consequence, but only one of them is supposed to exist in that dimension, so they're unstable. When they touch each other, Parallel Annie is destroyed.
The Movie utterly destroys this trope by having Yuuto grab three versions of Ryotaro out of the normal time stream, hitting them with a powerful sedative, and then having three of his four Imagin possess the unconscious Ryotaros, allowing all four primary versions of Den-O to exist at the same time and participate in the final battle.
Kamen Rider Fourze destroys this trope even so more than Den-O in the Cross Over film Movie Wars Ultimatum. When the five-years-later version of Gentaro Kisaragi comes back in time to our present, the very first thing he does is pull his past self aside (via tackle), say "Hi, me!", and ask to borrow his Transformation Trinketsince he destroyed the future version as a Secret Test of Character for one of his students. Present-Gentaro's first reaction is of course utter shock, but after he calms down he gladly loans the belt.
In the core Timemaster rules (from the defunct Pacesetter Games), it was impossible to meet up with yourself. Any attempt to travel to a time period you already occupied put you into a "Loop Trap" — you'd basically be "stuck" reliving the time covered by the loop over and over. Of course, you wouldn't realize this, because it would be the first time through every time. Better hope one of your teammates is willing to pull you out of the loop. Timetricks, a Timemaster supplement meant for more experienced groups, included a little gadget that would let you bypass a Loop Trap for a short period of time, assuming it worked.
Played with in Genius The Transgression. Interacting with your younger self is relatively safe compared to all the other stupid things you can do during a time-travel jaunt. However, going back in time to the same moment twice and coming into contact with your own time-traveling self is an excellent way to drive yourself all sorts of crazy — when the game describes an act as "about the stupidest thing you can do without a death ray and a bottle of tequila," you know it's a bad idea. It also handily prevents an "army of temporal duplicates" scenario from coming to pass.
In the obscure Dungeons & Dragons second edition SplatbookChronomancer, this is in effect for anyone who travels to a time period even close to when they already exist (either from their natural life or other time travel jaunts). Various forms of Clock Roaches will attempt to force a time traveler into a temporal frame they didn't previously exist in, some rather destructive. Eventually, should they avoid all those, a Timey-Wimey Ball will automatically force the offending time traveler to a point forward where it's no longer an issue.
Continuum'' not only inverts this, but expects this will happen and has rules (both etiquette and mechanical) on how to handle such an event (dubbed a "Gemini incident"). Remember to respect your elders.
Magic: The Gathering has rules in place to partially enforce the alternate-universes version of this, although whether the time-travel version is also enforced depends on whether you're a legendary creature or a planeswalker:
The "legend rule" applies to legendary creatures (or legendary permanents of any type). Legendary permanents represent unique, named characters, places, or objects rather than generic ones; the legend rule states that if a legendary permanent entering the battlefield shares a name with one already on the battlefield, both are sent to the graveyard. However, two legendary permanent cards could represent the same character without sharing a name. For example, Kamahl, Pit Fighter and Kamahl, Fist of Krosa represent the same character but have different names, so they could coexist on the battlefield.
The planeswalker card type has a similar rule to the legend rule, only it refers to a planeswalker's subtype; if a planeswalker entering the battlefield shares a subtype with one already on the battlefield, both are sent to the graveyard. This is because there are no generic planeswalker cards; each represents a specific character, and the subtype indicates which character is represented. This means that two planeswalker cards that represent the same character without being the same card still destroy each other. For example, a popular strategy to combat one-timeGame BreakerJace, the Mind Sculptor was to either remove him by casting his less-expensive version Jace Beleren, or use the fact that Jace Beleren is less expensive to put him into play before a Mind Sculptor is cast at all, meaning the other player would have to waste a Mind Sculptor card in order to clear the way for another one. Since both represent the same character (in game terms, since both are Planeswalker - Jace), they destroy each other by being in play at the same time.
This happens intermittently in Terranigma. When the 'Dark World' Ark meets the 'Light World' one's spirit, Ark is instantly killed and reincarnated as a baby with a mixture of the Light's spirit and the Dark's memories and personality. Later, his girlfriend Elle meets the Light World version of herself... and nothing happens. In fact, for a second it seems they're about to get into a Cat Fight.
In Shadow Of Destiny, multiple versions of the same person can be in the same room and even talk to each other without problem. Physically touching your past or future self, however, results in both of you ceasing to exist. This becomes a plot point in various possible endings.
In the small Interactive Fiction game All Things Devours, sighting yourself - or causing your earlier self to notice anything you didn't - causes the space-time continuum to go blert with the force of a nuke. This is a problem, as the main character is trying to destroy her prototypical time machine with a range of minutes, inside a military base that nobody could infiltrate alone. Fascinating choreography, shameless Trial-and-Error Gameplay.
In Legacy of Kain the moment when two incarnations of the Soul Reaver meet a paradox is created, and the resulting distortion of the timestream allows to make changes in history, which is usually written in stone, this happened four times in the games.
In Fate/stay night, it's explained that having copies of the same person present in the same time period causes a strain on reality that will manifest on both copies, as reality cannot truly distinguish between them. They will slowly start siphoning knowledge and skills from each other merely by being around each other, and eventually, one or the other has to go. This is compounded more directly in-story by the future copy trying to kill his past self in the hopes a Temporal Paradox will erase him from existence.
Subverted in one of the Sam & Max episodes, involving a lot of time travel. The duo meet their past selves from a year and a half earlier, putting them early in the previous season. It doesn't mess up the universe, but it does result in Sam and Max getting trapped in the past and having to relive the entire year and a half over again because the past versions of themselves steal the time machine.
Through the mixed-up and convoluted story of Sonic the Hedgehog (2006), this essentially takes place. There probably isn't a time where there aren't at least two Sonics/Silvers/Shadows running around at the same time, just in different locations. For example, late in the game, Sonic, Silver, and (strangely) Blaze find themselves on a cliff, while Doctor Eggman's Egg Carrier crashes into the side of a mountain, leaving Sonic to believe Elise is dead. Silver then suggests Sonic goes back in time to rescue her. While this happening, Sonic has already done so. He and Elise had already escaped the crash just as the carrier exploded.
Averted in Sonic Generations. The entire gimmick of the game revolves around both Classic and Modern Sonics (and they meet up rather quickly.) In addition, there is also a meeting up of Classic and Modern Tails and Classic and Modern Eggman - who both pilot the final boss.
In Onimusha 3: Demon Siege, player-Samanosuke comes back in time and sees his alternate-timeline self laying slain by Nobunaga. Touching his Oni Gauntlet merges the two together and the combined power from the paradox enables Nobunaga's defeat.
Used as a plot point in Super Robot Wars Z 2, albeit with dimensional travel rather than time. Under the physical laws of the multiverse, a person cannot travel to any parallel world where a version of themselves is already present; when the heroes from SRWZ 1 travel to the new world, the MazingerTeam and the Getter Team get left behind since the Z 2 world already has the Shin Mazinger and Getter Robo Armageddon versions of them. The rest of the heroes didn't realize this until they met the parallel Mazinger and Getter Teams and started asking questions about their allies, which naturally they couldn't answer.
Also used with a twist in regards to the Original Generation. In the Super Robot Wars Alpha timeline, Ingram Plisken creates an incredibly powerful robot called the Astranagant; however, in the Super Robot Wars Original Generation timeline, he instead upgrades his old R-Gun into the R-Gun Rivale, which is close to but not quite the same thing. The official reason given is that the Astranagant (and its pilot) are the Guardians of the Multiverse, so there can only ever be one Astranagant in all of existence, and not-quite-knockoffs like the R-Gun Rivale are the closest any other universe can get.
A non-paradox version of this in Fire Emblem Awakening is the stated reason the Kids From The Future leave once their present selves are born. The future children don't want to live with their present selves because it would negatively impact their childhoods. Averted for Lucina and Noire, as the former is already born while the latter stays with her present self to right her relationship with her mother.
Some levels in The Adventures of Shuggy generate clones of Shuggy at set intervals. If he makes contact with any of his clones, he dies and has to start the level over from the beginning.
Surprisingly averted in Blazblue, which most of its plot consists of time travelling. Hakumen, comes from an alternate timeline which was similar to the canon one the series takes place in, save for a few major details changed. He makes his way to the canon storyline by being flung back in time 100 years prior to the beginning of the first game and becoming one of the world's legendary heroes. His story path in the first game reveals he is the reincarnation of the series` deuteragonist, Jin Kisaragi. The two have encountered each other quite a few times throughout the series, but there doesn't seem to be any adverse affects of this trope happening.
"If some guy came up to me claiming to be my future self, I'd sense a setup and toss him overboard, figuring if he was me I would have expected that."
Meeting and hanging around with yourself in Timeless Space is generally considered a bad idea, since it means you've managed to escape once before (and returned later), and most denizens of the place would kill to find out how you did it (or to prevent their enemies from finding it out). Having a normal twin is even more dangerous, since people will just think you managed to escape, even though you never did.
One of the big issues in the first time travel arc of Casey and Andy, when several of the main characters end up back in the time when they were in high school. But it's easy to find the young Casey and Andy, since you only have to look for the biggest explosion.
Played straight (so far) in Sailor Sun as Honey (the Kid from the Future) seems to be unaware that her present self (Brady) was sent to live with her aunt shortly before Honey arrived in our time and it's implied that her mother may be deliberately keeping her in the dark.
Homestuck has no explicit taboo against multiple selves meeting each other, as long as their interactions form a Stable Time Loop. However, selves from splinter timelines are "doomed" and will inevitably be culled from existence: if you go back in time to prevent your past self from triggering the Bad Future you came from, don't expect to live long afterward.
In Frankie and Stein, Shelly claims that if they meet themselves in the future, their heads will explode. Later, she and Stein meet their future selves, and future Shelly disproves this theory and tells them to calm down, subverting this trope quite nicely.
One storyline in Dragon Tails involved the dragons travelling through time. At one point, they end up back in their forest a few years earlier, on Christmas day. Cornelius and Abijar want to go join their past selves to celebrate Christmas, at which point Enigma brings up this trope. It does not turn out as he planned.
The old webcomic Jamie and Nick featured the alternate dimension version. Traveling to an alternate dimension is always a bad idea, even if your counterpart is already dead. If your counterpart is alive, the universe will eventually notice and erase both of you from existence. If your counterpart is dead, the universe gets confused and gradually rewrites your memories with the memories of your other self.
In The Randomverse, Lex Luthor explains away a potential plot hole by explaining this trope. Lampshaded by Deadpool, of course.
The Ben 10: Alien Force episode "Paradox" has Kevin's car turned into a literal rust bucket by a time monster that ages anything it touches. At the end of the episode, Paradox gives them a new (old?) car, with the warning that it will "explode like antimatter" if it comes into contact with anything else from 1976. However, earlier in the episode, Paradox spoke to an alternate universe version of himself without any ill effects - granted, they did not shake hands or anything, but still... note Word of God is that Paradox was joking about Kevin's car.
One episode of Buzz Lightyear of Star Command has the heroes encounter their alternates from another dimension (where the biggest difference was that Buzz himself was evil). XR warns the team to avoid touching their counterparts — just as the two Boosters greet each other with a hug. When nothing happens, XR remarks on the trope not applying.
In the Futurama movie Bender's Big Score, we learn that duplicates of persons or objects created by time travel are automatically doomed, as a form of Equivalent Exchange. This becomes a plot point as we learn that Leela's new fiancee is another version of Fry, and calls off the wedding rather than subject Leela to the grief caused by his ultimate demise. And sure enough, he dies at the end in a Heroic Sacrifice. At the very end, hundreds of Bender duplicates appear and as they explode one by one, the sheer volume of temporal anomalies leads to a rupture in the universe - and a Sequel Hook for the next movie. This is almost a subversion as until time corrects the paradox by destroying the duplicate duplicates are free to touch and interact with each other.
Scammer Alien: I met my past self in a bar for a drink, one thing led to another, and we ended up back at my place. Or should I say, "our" place. (passionately embraces duplicate) Everyone Else: Ewwwww!!! Scammer Alien: Oh come on, you prudes!.
In the season finale of Superjail!, the Warden goes back in time to his own trial to save himself, and by the simple act of hugging his past self, causes the very fabric of reality to completely and utterly fall apart.
On The Penguins of Madagascar, Kowalski invents a time machine, but a future Kowalski has come to get Private to stop him, warning him that if both Kowalskis were to ever meet, it would cause a rip in the space-time continuum. Also, a third Kowalski has come to get Skipper to keep the time machine from being destroyed. When the two future Kowaslkis meet, they reassure the others that it's okay, as long as the original Kowalski doesn't see them. And that's when original Kowalski sees them, causing the space-time rupture that led them all here in the first place.
In Taz-Mania, Marvin the Martian's team-up with Taz. For whatever reason, hooking Taz up to some big gyroscope thing allows him to spin them through time. Marvin is always cautioning him not to meet himself, as "It results in an Earth-Shattering Kaboom." Of course, he forgets this advice. The result was somewhat similar to the ending of that Tiny Toon Adventures episode where Buster, Hamton and Plucky drank beer.
Another Looney Tunes short has a humorous non-time travel invocation of the trope. Playing hide and seek with Prissy's little "egghead" son, Foghorn Leghorn hides in a storage bin. The young genius plays with a slide rule, scribbles down some calculations, marks an "X" on the ground, and proceeds to extract Foggy from that very spot with a shovel. Foghorn protests, but the little Einstein just points to his calculations. As the scene ends, Foggy passes the bin in which he had hidden, and starts to curiously lift the lid, but stops himself.
Foghorn: No, I'd better not look. I just might be in there!
In the DCAU, specifically in Justice League, this is the explanation used by Vandal Savage for why he doesn't go to the past and stop himself from destroying the world; considering that Vandal Savage is an immortal caveman who has lived out virtually all of human history, that means he effectively can't time travel at all. He specifically says that it's a result of his particular time machine's design rather than an immutable law of the universe. In fact, Batman travels to the future and meets the Batman Beyond version of himself in a later episode, with no consequences. Admittedly, time outright collapsed in that episode, so "no consequences" is a bit of a relative term. In the short term, at least, it turned out fine.
Other than the alterations Vandal wanted to cause, his meeting of his past self didn't seem to cause any major damage.
The Batman Beyond version of Batman had no recollection of his younger self's perspective of their meeting or the events that led to the meeting. That said, the timestream was already pretty much in flux by that point for unrelated reasons.
Static traveled to Batman Beyond and briefly met his future self. No Earth-Shattering Kaboom, and his future self didn't seem the least bit surprised.
Bump in the Night: Mr. Bumpy unwittingly insulted Squishington, who locked himself away as a result of it. Bumpy then went back in time to prevent that but failed. Then he went back again and failed again. The three Bumpys then got an advice from Future Squishington and apologized to their friend. When Squishington asked the Bumpys why there were three of them, they disintegrated because they couldn't agree on which ones had to leave.
Kim Possible: In "A Sitch in Time", when Kim defeated Drakken, Killigan and Monkey Fist, Shego got a visit from her future self, who advised her to steal the time monkey and use it to take over the world. Past Shego first assumed Drakken cloned her.
Lilo & Stitch: The Series: Jumba invented a time machine and installed a paradox inhibitor to prevent its users from meeting past selves. The users would basically rewind time. Lilo and Stitch once forgot to use the inhibitor and met their past selves but no disaster seems to have happened from it.
"Father Time" had Timmy and his fairies travel back to the 1970's. Timmy found out a young Bill Gates was Cosmo and Wanda's godchild. Cosmo and Wanda met their past selves but nothing bad happened from it. When he was back in present time, a future Timmy showed up with his Cosmo and Wanda, destroyed the remains of a trophy and told present Timmy he'd thank him for it. One can only wonder why.
"The Secret Origin of Denzel Crocker" had Timmy and his fairies going back to 1972 and Timmy found out Cosmo and Wanda were Crocker's fairy godparents back then. It's unknown if past Cosmo and Wanda recognized their present selves, but past Jorgen recognized present Jorgen.
"Channel Chasers": Coming from a Bad Future, an adult Timmy went back to his childhood to prevent Vicky from taking over the world.
In The Smurfs episode "No Time For Smurfs", Handy accidentally creates temporal copies of himself, Brainy, Clumsy, and Smurfette when they mess around with Father Time's Sands Of Time hourglass by first causing time to go backwards and then causing time to repeat previous events such as their discovery of Father Time's workshop. Averted in that nothing disastrous happens, but the temporal copies do inform Papa Smurf that they have seen what they think are copies of themselves in a cave. Fortunately, by the end of the episode, Father Time and Mother Nature straighten the whole mess out, and the temporal copies vanish.
This Trope is Defied in the Dexter's Laboratory movie Ego Trip, where the Dexter everyone's familiar with uses his time machine to form a team consisting of himself and his Teenage, Adult, and Elderly selves to Set Right What Once Went Wrong, courtesy of Mandark's tampering. There was no Earth-Shattering Kaboom, despite the fact that Mandark used the same trick to fight them. In fact, after Dee Dee intervenes, she accidentally crates a future utopia.
In the 2014 Mr. Peabody & Sherman movie, going back to meet yourself is technically okay, but coming into contact with your past/future self causes the two to combine into one and the timeline to dissolve, dragging various historical figures and landmarks into the present. When Mr. Peabody realizes Sherman is at risk of causing this paradox, he runs in and shouts "Sherman, I have to stop you from touchingyourself!" which doesn't bode well for his social services evaluation.
This is literally the case with fermionsnote a class of fundamental particle that includes electrons, also can refer to anything with an odd number of fermions. Two of the same fermion cannot exist in the same place. It's inverted with bosons, which are more likely to exist in the same place than predicted by chance. Time travel is unnecessary here, since the way quantum physics works, any two particles with the same properties are, in fact, the same particle. Interestingly, since chemistry is based on this fact, somehow getting ahold of truly different electrons would be disastrous, if far from reality-destroying.
Using special relativity and the last comment, it's easy to make this situation happen in real life, and show that it's not a problem. By moving one particle faster, it can be made to pass through time at a different rate. Since both are the same particle, but more time passed for one, then one of them is a future version of the same particle. They interact as normal, showing that they are, in fact, the same particle, and being Just One Second Out of Sync is not an attribute that exists on a fundamental level.