Doppelganger Crossover

""So," Chekov clapped his hands in eagerness, "who's the problem?" When Riker pointed down at the deck, he blinked and said, "Oh, there he is." He knelt down and rolled the still-senseless Bester over, checking him for weapons. "Rather a handsome devil, isn't he?" "We hadn't noticed," Worf growled."

- Pavel Chekov taking Alfred Bester into custody, during the Undocumented Features story "A Day of Infamy".

Let's say you're a fanfic writer. Let's make another assumption and say that you like a particular actor and everything they've ever been in, because they were in it.

It's entirely possible that you may write a Doppelganger Crossover, a story in which two characters played by the same actor meet each other. Or that they are each other. In other words, a crossover which is only happening because the two sources share an actor.

You may desperately want Riker to meet Xanatos, or Ranma to be a reincarnation of Inuyasha, or you might just want to see what happens when John Crichton takes the place of Cameron Mitchell, and don't even start on what would happen if Han Solo bumped into Henry Jones Jr.. The end result, however is the same. A Doppelganger Crossover.

On occasion, this can be one part of a Massive Multiplayer Crossover. Compare I Want You to Meet An Old Friend of Mine. Compare/contrast the Folgers Crossover.

Anime and Manga

 * When Ranma ½ crosses over with Ah! My Goddess, it is inevitable that Belldandy and Kasumi (both sweet, innocent, domestic characters voiced by Kikuko Inoue) sit down to trade recipes, or better yet, collaborate on a meal.
 * Subverted in Chris Davies' Mega Crossover Together Again 1996 Chapter, where it's revealed that Kasumi is a reincarnated inhabitant of the Silver Millennium with some very strong, very negative opinions about "the monsters who call themselves gods" (to use her words from the story), and who is outright hostile to Belldandy.
 * However, they seem to be getting along much better in another story in that series, Together Again 2937 Chapter.
 * There was also at least one Ranma fanfic crossing over with Magic Knight Rayearth, on the basis that Hikaru Shidou looks like Girl-type Ranma. A Duet of Pigtails is the most famous example here.
 * Plenty of fanfic also involving Lina Inverse meeting up with female Ranma. Or the latter and Rei Ayanami. There was even one with Ai Amano. Blame Megumi Hayashibara and her amazingly prolific career.
 * And don't forget the some-odd Ranma/Inuyasha crossovers where Ranma invariably turns out to be Inuyasha's reincarnation. They're up in the article itself for a reason.
 * Yeah, the page image comes from advertising It's A Rumic World, a compilation of short works by Rumiko Takahashi, by using the main characters of her three most famous series: Inuyasha, Ranma ½, and Urusei Yatsura. Inuyasha even lampshades this trope by demanding to know why Ranma has his voice.
 * A target of frequent fic speculation is voice actress Kotono Mitsuishi, whose incredibly broad portfolio of roles offers a vast range of possibilities: imagine, if you will, an intersection of any two or more of Misato Katsuragi, Mink, Usagi Tsukino, Excel Excel, Christie, Murrue Ramius, Cathy Taymor, Gracia Hughes, Kagura Soma, Mireille Bouquet, Juri Arisugawa, Ebichu and Birdy Cephon Altirra — just to name a few.
 * Super Robot Wars parodies this anytime Misato Katsuragi and Amuro Ray show up in the same game. Why? Because they're Sailor Moon and Tuxedo Kamen!
 * Busou Renkin fanfic writers like to crossover with Rurouni Kenshin, usually making Kazuki into Kenshin's son.
 * Higurashi no Naku Koro ni and Magical Girl Lyrical Nanoha? Since Rika and Nanoha share a voice actress, there have been fan videos that involve the two.
 * There is at least one Fullmetal Alchemist/Ouran High School Host Club crossover fan fic which exists purely because the English dubs of both anime have the same voice cast.

Film

 * Harrison Ford has such a career with fingers in the pies of so many fandoms that he is almost a too-obvious go-to person for this fanfic trope. What happens when Jack Ryan tries to rescue the president aboard Air Force One? What if Lieutenant Deckard is chasing The Fugitive? What if Indiana Jones found the wreckage of the Millennium Falcon in the Northwest United States while looking for Bigfoot? (That one is an official, albeit non-canon, comic story, mind you.) And most importantly, who would win in a fight?
 * The Harry Potter and Twilight series both feature actor Robert Pattinson in their respective film adaptations, although it would take some rather violent Hand Waving to make Cedric Diggory and Edward Cullen the same person. This has mostly been fodder for the MuggleNet Caption Contest.
 * That said, Cedric seems to have become a much more popular character in Harry Potter fanfiction since the first Twilight movie.
 * Bellatrix Lestrange, meet Mrs. Lovett and The Red Queen.
 * There are fics out there about Legolas meeting Will Turner and/or Balian and/or Orlando Bloom himself.

Live-Action TV

 * Let's just say that the relationships between Captain John Hart and William The Bloody really run the gamut from being same person to being enemies, friends, lovers or basically anything else you can imagine.
 * There are umptynine Life On Mars fics that reveal that Sam Tyler is... in fact, the Master.
 * Stargate SG-1. MacGyver. There are too many of these fics, far too many JackGuyvers.
 * And, as mentioned above, there are waaay too many Starscape fics. Also (but not so often) called Fargate.
 * Buffy the Vampire Slayer/Firefly crossovers have at least three special variations unique to them, one where Xander initially panics upon meeting Mal Reynolds because of the latter's resemblance to Caleb, and one where cast members from Angel mistake Zoe for Jasmine or Jayne for Hamilton.
 * Related to above, there are a number of Firefly/The Sarah Connor Chronicles crossovers that usually feature Cameron being mistaken for River, often with Alliance soldiers getting curb stomped.
 * Philip Akin appeared as a regular in the first season of War of the Worlds, Adrian Paul in the second. Since both actors also appeared in Highlander the Series, crossovers make up a sizable proportion of War of the Worlds fanfic, either with Kincaid as a cover identity for Macleod or one being mistaken for the other.
 * Captain Simon Banks has been mistaken for the Immortal Carl Robinson. Similarly, a certain Really Old Guy either gets mistaken for or actually is a certain serial killer.
 * Methos gets around a lot in general.
 * Andromeda all but canonized the notion that Dylan Hunt is the reincarnation of Hercules.
 * Masi Oka portrayed lab-guy Franklin on Scrubs and now portrays Hiro Nakamura on Heroes, a fact picked up on in one or two fanworks.
 * Buffy the Vampire Slayer and Veronica Mars share enough actors to make an arsenal of this, albeit most of them in minor roles in the latter. And that's without getting to the fact that Veronica dated Iceman and Mac dated The Flash. It's superhero central, guys.
 * One can only speculate at just what the hell happened to Badger once the crew left him to make him straighten up, learn the law and start ste— no, wait, that one's already covered. Still, going from kinda-Cockney crime boss to President of the Colonies, pretty impressive feat.
 * The first "Echo is Faith" fics started showing up less than 24 hours after Dollhouse was announced—i.e., more than a year before the first episode aired. Amy Acker getting cast didn't help.
 * There seem to be quite a few Supernatural and Dark Angel crossover fics. Their ubiquity isn't surprising, as Jensen's characters in Dark Angel, the twins Ben and Alec, were genetically engineered, and thus could conceivably be clones of Dean. In light of that, and the Twin Threesome Fantasy trope, it was probably inevitable. However, Dark Angel is set Twenty Minutes Into the Future. So, if Dean isn't going to be in his forties, time travel or alteration to canon is necessary. Or heck, they could be parallel universes. One form of fic which is surprisingly common despite messing heavily with the timeline is the story in which Alec left Seattle after "Freak Nation" and became Dean.
 * Interestingly subverted in a few House fanfics that somehow add Jeeves to the mix instead of Bertie.
 * Played amazingly straight in Jeeves and the Evil Twin, a Jeeves and Wooster fanfic in which Jeeves comes home from vacation to find that Jack Dalliard (one of Stephen Fry's recurring characters in A Bit of Fry and Laurie) has been not only impersonating him but abusing and raping Bertie.
 * Bringing this trope to new extremes, there's Shatnerquake, where every single character that William Shatner has ever played comes along to wail on the man himself.
 * A fanfic that crossed over JAG with The Pretender had Lt. Cmdr. Rabb mistake Miss Parker for his former partner Lt. Caitlin Pike. Both Miss Parker and Lt. Pike were played by Andrea Parker.
 * Carly/Megan. A few people go so far as to add Summer Hathaway, from School of Rock, into a Miranda Cosgrove Doppelganger One True Threesome.
 * The fanfic A Resemblance had NCIS's medical examiner Dr. Donald Mallard, better known as Ducky, mistake Rizzoli and Isles's medical examiner Dr. Maura Isles for his friend Kate (he was involved in a small accident). It can be assumed to be Kate Todd from the first two seasons because Kate just happens to be played by the same actress who plays one of the title characters from Rizzoli & Isles. Any guesses?
 * Claude from Heroes being secretly the Ninth Doctor was a very popular idea for a time.
 * Ever wonder why there's a [[Criminal Minds]/Dharma and Greg crossover section over at Fanfiction.net]? Because Thomas Gibson was in both shows, inspiring fics where Agent Hotchner (a.k.a. "Hotch") is actually Greg in disguise and vice versa.
 * This has also happened with Sue Sylvester, Sam's mother, and Reid's mother due to Jane Lynch playing all three characters.
 * It's popular in fan-works to suggest that Richard Castle is either a distant ancestor of or is in some Timey-Wimey Ball-way connected to Malcolm Reynolds.
 * Apart from the natural "Booth is a sanshued Angel" fics, there have been a couple of Bones/Angel fics where Angel somehow meets Booth.
 * At least one Anthony Head doppelgänger Merlin/Buffy the Vampire Slayer fic exists with Uther and Giles meeting.
 * It can be rather difficult to watch My Life in Film and not see Jones as Jim Moriarty. Especially when he starts talking about burying people in back gardens.

Video Games

 * While TEEN FORTRESS 2 isn't explicitly a Doppelganger Crossover, it does substitute Principal GLaDOS for the Announcer.
 * Not to mention, in said fic Principal GLaDOS is actually Caroline's mother, and not the woman herself.
 * Speaking of Valve, they also played it straight in the canon works: they originally had an idea for a scientist called Greg to be playing off Cave Johnson in Portal 2, but decided it was idiotic to hire a voice actor for just a few lines. Instead, they decided to use GLaDOS's voice actor for what we now know as Caroline, and the fact that is history.

Western Animation

 * In a Ben 10: Ultimate Alien crossover with the DC Universe, Gwen fights Slade, who remarks that "You remind me of someone I knew a long time ago, she was a lot like you. You Even sound like her too... let's just say that someone with the power of Geokinesis used to work for me." Considering the fact that Ashley Johnson plays both Gwen and Terra, it's pretty fun, though they haven't met each other yet.

Multimedia

 * The Official Fanfiction University of Middle-Earth contained a chapter where the "teachers" (read: cast of The Lord of the Rings) were all replaced by other characters played by the same actors.
 * Wasn't that the fault of the original two Protectors of the Plot Continuum, who were visiting? Or was that a different disaster event?
 * Done in reverse, with the official X-Men/Star Trek: The Next Generation crossover which featured Captain Picard meeting Professor Xavier, but came out before the first X-Men movie.
 * Aside from the above example of Xanatos and Riker, Gargoyles and Star Trek the Next Generation shared Marina Sirtis, Michael Dorn, Brent Spiner, Le Var Burton, and Colm Meaney. Cameos from other Star Trek series included Kate Mulgrew as fairy queen Titania (often disguised as a human scientist) and Nichelle Nichols as the female lead's mother. Crossovers milked this for all it was worth.
 * There is a Dr. Horrible's Sing-Along Blog/How I Met Your Mother crossfic where Barney Stinson seduces Dr. Horrible.
 * As well as countless Dr. Horrible's Sing-Along Blog/How I Met Your Mother crossovers where Barney secretly is Dr Horrible...
 * This fic, From Prometheus to 12 plays with the concept that Felix Gaeta and L are the same person.
 * There is a fanfic from Sarmy, a Sylar and Zachary Quinto fansite, which crosses almost every role Zachary Quinto has ever played. Sylar, Spock, Sasan, Adam Kaufman, and tons of guest appearances feature in the fic.
 * Popular in Doctor Who fandom, especially This Time Round. In particular, the Minister of Chance from Death Comes To Time is secretly every character Stephen Fry has ever played, especially Charles Prentiss.
 * The fic "In the Cafe of Reasonable Comfort" does something similar with.
 * There now exists an entire genre of Star Trek fiction where Bones is actually The Doom Marine, or at the very least, his something-great grandson.
 * In the Operations Manual for West End Games' Ghostbusters: A Frightfully Cheerful Roleplaying Game, published in 1986, contains an adventure titled "The Couch Potato," in which (assuming the players are using the characters from the movie) Dr. Peter Venkman meets a character named Dr. Hunter Panama. The manual notes right away that Doctor Panama is based on Dr. Hunter Thompson and suggests watching the movie "Where the Buffalo Roam," in which he is played by Bill Murray (who, in case you can't guess, also played Dr. Venkman), to give you ideas on how to play the character. The manual also remarks that two Bill Murray characters playing off each other appeals to their schizophrenia.
 * Given how many hundreds of sources feed into it, it's inevitable in Undocumented Features. It even gets lampshaded when it happens, as the page quote demonstrates.