When you're meeting an alternate universe version/clone/zombie/etc of someone, you're essentially dealing with a stranger. But because of your experience with your version, you may not feel that way.
For better or for worse, some people trigger very powerful emotional reactions in others. What specifically is being triggered varies — affection, protection, disgust, mistrust. However, it's very possible for the person triggering the response to be someone other than the person who originally provoked it.
Maybe the hero is meeting his Dead Little Sister's alternate-universe counterpart. Maybe he's reluctantly making himself team up with a good-aligned clone of his arch-nemesis. Maybe he's having to shoot one of his True Companions who has succumbed to The Virus and effectively stopped being the person he knew. The point is that he's still having an emotional reaction to the person because of who they "are" — even though they aren't.
In some cases, this may not have any negative effects; the recipient is similar enough to the original target that it's still the correct reaction. However, sometimes (such as if the morality of the counterpart is inverted) things may not work out so well.
Replacement Goldfish is similar, but in those cases the individual getting the benefit of the character's previous relationship are not in any way the original themselves, though they may resemble them in some fashion. Also, this trope may last a shorter amount of time (for instance, if the hero meets the Alternate Universe version of his dead sister but then has to go back to his universe).
A subtrope of Psychological Projection. Related to Unbalanced by Rival's Kid, Close to Home, Doppelgänger Replacement Love Interest, and Reincarnation Romance. It's also the reason why the Shapeshifter Guilt Trip is sometimes effective.
- In Yu-Gi-Oh! , the "Dawn of the Duel" arc has Yami Yugi/The Pharaoh gets whisked into a world resembling his past life of Ancient Egypt. There he sees that his royal subjects are all past reincarnations of many people he knows in the present, and initially had trouble getting them any differently. For instance, his chief adviser is Shimon and the doppelganger of Yugi's grandfather, who the Pharaoh still sees as a father figure, while Priest Seto still ended up becoming a worthy rival to him of sorts.
- In Dragon Ball,
- Goku's reaction to fighting Copy-Vegeta seems to imply this. Even knowing this isn't really his rival, Goku just can't help enjoying the fight against this Vegeta. It helps that Copy-Vegeta completely absorbed the real Vegeta's power, personality, and memory, so Goku was pretty much fighting the real Vegeta colored purple.
- Vegeta seems to treat fighting Goku Black as another chance to beat on Kakarot, always eagerly snatching the chance to pound his rival's doppelganger.
- Detective Conan has an extremely unusual example. In "Murderer, Shinichi Kudo", in an attempt to frame Shinichi Kudo, a criminal underwent plastic surgery to take on Shinichi's likeness and then put himself into a murder situation to get Shinichi convicted. However what gives away the culprit's identity; in his home a smashed mirror was found. The reason for it? He had taken on the face of a man he hated and seeing that same face in the mirror led him to smash it in anger. This guy treated the doppelganger the same way he would to the real person when said doppelganger was himself.
- In the Darkwing Duck arc Crisis on Infinite Darkwings, Quiverwing Darkwing acts as an affectionate and somewhat mentorly figure to Gosalyn Prime. He used to have his own version of Gosalyn, who was his sidekick, but she died at some point prior to the arc (hence why he goes by her superhero identity).
- Subverted in The Multiversity: Pax Americana; after Captain Atom impulsively kills his dog in an effort to comprehend how it works, he then creates an exact replica, but can't bring himself to love it because he knows it's not the same dog.
- In the Stardoc novel Plague of Memory, Cherijo encounters what appears to be the reincarnation of her long-dead first husband Kao Torin, which understandably complicates her mission. It turns out to be a Hsktskt that was put through Magic Plastic Surgery using a DNA sample from Kao's corpse.
- Shota the witch woman from the Sword of Truth series first meets the protagonist while shapeshifted into his long dead mother. Though he isn't fooled, it's difficult for him to remain angry at her in that form.
- Still grieving over the death of Laurel Lance, Oliver is greatly affected by the appearance of her Evil Doppelgänger Black Siren. Despite his continual denial, it's apparent he sees her as the original Laurel and a second chance to save her.
- Detective Lance, Laurel's father, has a much stronger emotional reaction. In fact, he has an arc where he kidnaps her to try to convince her that she can be his daughter if they both believe hard enough.
- The Flash:
- This is how Team Flash reacted when Harry Wells from Earth-2 first made his presence known. Being the Alternate Self version of the man who Eobard Thawne was impersonating made it very difficult for Team Flash to trust him, despite Harry's constant reminders that he was not Thawne.
- Earth-3 Jay Garrick is the doppelganger of Barry Allen's father, Henry. Barry is visibly emotionally overcome trying to deal with someone who looks just like his recently murdered father. Jay later learns Barry's situation. He feels bad for Barry, but also reminds him that he is not Henry and wouldn't make a very good Replacement Goldfish.
- This also occurs with HR Wells from Earth-19, the third Wells to be a member of Team Flash. Despite the fact that he was incompetent and unintelligent compared to his predecessors, Team Flash still accepts him as a team member. As Cisco explains, the other Wells were geniuses who they depended on, while with HR it was the inverse. So after everything that Harrison Wells had done for them, Cisco wanted "to be there for Harrison Wells."
- In the Season 2 finale, Zoom was Dragged Off to Hell where he was subjected to a Fate Worse than Death, becoming Black Flash, a zombie-like speedster with no traces of the mind of his former self. However when Barry and Killer Frost encounter Black Flash they still call him by his old name, and in Barry's case is evidently still terrified of his former nemesis.
- In Legends of Tomorrow, the Earth-X version of Leonard Snart, Leo, is an unrelenting do-gooder hero, which is stomach turning for his Earth-1 partner in crime Mick. Leo sees Mick as a Replacement Goldfish to his late partner Mickey, trying to help him out of his alcoholism and make him a better person, but this only seeks to infuriate Mick.
- Crisis on Earth-X:
- The Earth-X Version of Prometheus is the doppelganger of Oliver's dead best friend, Tommy, who died while the two were still having a fallout over Oliver being a secret vigilante. Oliver deeply wants to believe in the capacity for someone with his friend's face to change, but Prometheus, who was raised an unrepentant Nazi, uses Oliver's nostalgia and love for his friend to mock and manipulate him before deploying a cyanide capsule that forces Oliver to helplessly watch his best friend die a second time.
- When the Earth-1 heroes get captured and sent to Earth-X for execution, Earth-X Quentin Lance expresses difficulty killing Oliver, the man with the face of the fuhrer. So he opts to have a bag put over Oliver's head to make the act easier for him.
- Earth-X Quentin Lance also appears to be sentimental when he sees Earth-1 Sara, the doppelganger of his late daughter. He asks why someone like her was being held captive, to which Sara explains she likes both men and women. Lance mentions how Sara was just like his daughter and adds that he expunged said filth from his bloodline, having shunned her for being bisexual.
- In Doctor Who,
- This is explored in the two-parter "The Rebel Flesh" and "The Almost People". The Doctor comes upon a factory where employers use "flesh avatars" for dangerous work. But trouble starts when the avatars or "gangers" become sentient and start protesting their individuality to their originals. The episodes explores if the gangers should be considered real or not, with the Doctor insisting they are while the factory workers believe they are not. The Doctor experiments this using his ganger, by switching his Identical Twin ID Tag and seeing if Amy (who knows the Doctor better than anyone else) could notice the change. Turns out she couldn't, proving the ganger and the Doctor were one and the same person.
- In "The Zygon Invasion", the shape-shifting Zygons specifically use this against the humans. When a squad of soldiers surround a church full of Zygons, one Zygon comes out taking the form of one of the soldier's mother, constantly pleading to him in a heartfelt way. The soldier cannot bring himself to kill what looks like his mother, and when other Zygons emerge as the loved ones of the other soldiers, they all agree to head into the church, where the Zygons promptly slaughter them. Furthermore, the Zygon commander, Bonnie, was impersonating Clara, which is implied to be one of the reasons why the Doctor was being very forgiving to her and wanted to help her pull a Heel–Face Turn.
- The Outer Limits (1995), "Second Soul": A man's wife becomes a donor for an alien race that needs to possess dead humans to live. The alien is not happy about his persistent interest in her.
- Star Trek:
- Star Trek: The Original Series:
- In "Shore Leave", Kirk knows the "Ruth" and "Finnegan" he meets cannot be the real deals (for one thing, Finnegan hasn't aged since the Academy) but he still enjoys indulging his emotions with each of the duplicates (having a romantic time with Ruth and whaling on Finnegan).
- Implied in "Mirror, Mirror". Kirk shows more softness than one would expect toward the Mirror Spock, who (despite being better than Mirror Kirk) is as ruthless as one would expect from the Mirrorverse. This probably has something to do with his close relationship to his own Spock. Kirk even comments on how alike they are.
- Star Trek: The Next Generation: In the episode "Parallels" (where Worf encounters many alternate universes) the Primeverse Enterprise briefly receives a transmission from an alternate-universe version of Riker, now in command after Picard died during the events of "The Best of Both Worlds". He expresses his happiness to see Picard, saying that it's been a while.
- Star Trek: Deep Space Nine:
- Enforced in "Rejoined" as an allegory to closeted homosexuality. The episode has Jadzia Dax work with Lenara Kahn, another joined Trill. The Dax and Kahn symbionts were married in previous lifetimes, and the two women find that they're still strongly attracted to one another despite "reassociation" being a taboo in Trill culture. They ultimately do not stay together due to societal pressure.
- When Jake Sisko meets the Mirror Universe counterpart of his deceased mother, he treats her as if she really were his mother. In return, she starts to view him as the son she never had.
- Star Trek: The Original Series:
- Stargate SG-1: After hosting the Tok'ra symbiont Jolinar in "In the Line of Duty", Samantha Carter inherits some of Jolinar's memories, which includes a romantic attraction to the Tok'ra operative Martouf, lover of Jolinar's previous host Rosha. Also inverted: Jolinar's memories complicate Sam's preexisting Unresolved Sexual Tension with the Tollan diplomat Narim, with Sam telling him she's holding off on dating until she's more sure whether she's feeling things herself or because of Jolinar.
- Sam and Dean find themselves sucked into an Apocalyptic Earth-2. While there, they meet that Earth's version of Bobby. Earth-1 Bobby was their father figure and died suddenly and tragically so it shakes the brothers to see him alive on this Earth. However, because of choices made by their parents, Sam and Dean were never born on Earth-2, so he has no such attachment to them.
- Jack's mother dies giving birth to him, so he's always felt something was missing, having been able to communicate telepathically with his mother in utero but never getting to meet her face to face. While working a case, he and the brothers encounter a shapeshifter who works as a bereavement counselor. She is able to use the brothers' memories of Kelly to take her form and give Jack a chance to "speak" to the mother he was never able to meet. Jack is thrilled to finally be able to hug her and thank her for protecting him.
- Sword Art Online:
- In Hollow Fragment, the video game adaptation of the series, Kirito encounters a Hollow Data version of Sachi, his deceased first love who he failed to save. Although suspecting she's not real, Kirito is hellbent on protecting her at all costs so she won't die again. Eventually Sachi reveals she is indeed not the real Sachi. She also remembers the words the real Sachi never got to say to Kirito, which leaves him distraught and shaken, but also giving him closure.
- Accel World VS Sword Art Online: Millennium Twilight, due to the time travel shenanigans of the game and the fact the game continuity is an Alternate Timeline to the canon continuity, Kirito ends up meeting a version of Eugeo from another timeline (who is implied to be the canon version). Upon seeing Kirito, Eugeo mistakes him for his Kirito and tries to get him to remember, only to be disappointed that this Kirito has no memories of their time togethernote . However after having a friendly duel with Kirito to test him, Eugeo realizes that while this is a different Kirito, it is still Kirito who fights the exact same way. He then takes up on Kirito's offer to hang out and party with him until he's able to return to his own timeline.
- The Witcher 3 features the doppeler Du-Du who can take on the form of anyone he pleases. In a time where anything not 100% mundane can lead to being burned at the stake, Du-Du tends to mimic people who are either very esteemed or very feared. This nearly gets him killed when he chooses to shapeshift Whoreson jr, one of the city's underbosses. Ciri, a ridiculously overpowered half-elf, and her father-figure, protagonist and renowned witcher, Geralt, nearly murder him as Whoreson, Jr. tried to kill Ciri the last time the two crossed paths. Du-Du is forced to reveal his true form to avoid being murdered by his own friends.
- In Homestuck, Dave spend much of the Intermission being very anxious/nervous about meeting Dirk, an alternate universe version of his "older brother" who raised him. His nervousness reflects his insecurity and fear towards his enigmatic and abusive "older brother", and Dave harbors similar feelings towards the alternate universe version, even though he is the same age as Dave and has none of the same history. When they ultimately meet, Dave uses Dirk to work out his issues with the older version, specifically his use of irony to pretend that he wasn't in a deeply unhealthy relationship.
- In the Darkwing Duck episode "Life, the Negaverse, and Everything", Darkwing finds himself in an alternate universe. He immediately becomes as fiercely protective of Nega-Gosalyn as he is of his own.
- In Justice League, the Justice Lord version of Batman panics when Flash fakes flatlining by speeding up his heart rate, because the Justice Lord universe's version of Flash was killed by Lex Luthor.
- In Phineas And Ferb The Movie Across The Second Dimension, the Phineas and Ferb from the second dimension get really excited to see the first dimension's Perry, hugging him for a few more seconds even after finding out he's not their Perry, who'd disappeared ages ago.