http://tvtropes.org/pmwiki/pub/images/Picture_5.png
[[caption-width:428:Seems a bit like [[TalkingToHimself talking to yourself]].]]

-->''"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.''
--> -- [[StarTrekTheOriginalSeries Pavel Chekov]] taking [[BabylonFive Alfred Bester]] into custody, during the ''Fanfic/UndocumentedFeatures'' story [[http://www.eyrie.net/UF/ "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 DopplegangerCrossover, 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 [[StarTrek Riker]] to meet [[{{Gargoyles}} Xanatos]], or [[RanmaOneHalf Ranma]] to be a reincarnation of [[InuYasha Inuyasha]], or you might just want to see what happens when [[FarScape John Crichton]] takes the place of [[{{Stargate}} Cameron Mitchell]], and don't even ''start'' on what would happen if [[StarWars Han Solo]] bumped into [[IndianaJones Henry Jones Jr.]]. The end result, however is the same. A Doppleganger Crossover.

On occasion, this can be one part of a MassiveMultiplayerCrossover. Compare {{I Want You To Meet An Old Friend Of Mine}}.
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!!Examples:

* The obligatory ''[[RanmaOneHalf Ranma 1/2]]'' example involves a crossover with ''AhMyGoddess'' -- in which it is inevitable that Belldandy and Kasumi (both sweet, innocent, domestic characters voiced by KikukoInoue) sit down to trade recipes, or better yet, collaborate on a meal.
** Subverted in Chris Davies' MegaCrossover ''Together Again 1996 Chapter'', where it's revealed that Kasumi is a reincarnated inhabitant of the [[SailorMoon 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''.
** The Boss from ''MetalGear'' was also voiced by the same person. This troper shudders in horrified morbid anticipation.
*** Nah, I bet nearly all of them only care for the English ''Metal Gear'' voice cast. Which in turn means that they can stick the Boss with [[SpongeBobSquarePants Pearl Krabs]]... and that'd be even worse.
** There was also at least one ''Ranma'' fanfic crossing over with ''MagicKnightRayearth'', on the basis that Hikaru Shidou looks like Girl-type Ranma.
** Plenty of fanfic also involving [[{{Slayers}} Lina Inverse]] meeting up with female Ranma. Or the latter and [[NeonGenesisEvangelion Rei Ayanami]]. There was even one with [[VideoGirlAi Ai Amano]]. Blame MegumiHayashibara 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 pic comes from advertising ''RumicWorld'' by using the main characters of RumikoTakahashi's three most famous series: Inuyasha, Ranma, and UruseiYatsura. Inuyasha even [[LampshadeHanging lampshades this trope]] by demanding to know [[KappeiYamaguchi why Ranma has his voice]].
* 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 [[TomClancy Jack Ryan]] tries to rescue the president aboard AirForceOne? What if [[BladeRunner Lieutenant Deckard]] is chasing TheFugitive? What if IndianaJones found the wreckage of the ''[[StarWars Millenium Falcon]]'' in the Northwest United States while looking for Bigfoot? (That one is an ''official'', non-canon comic story, mind you.) And most importantly, [[UltimateShowdownOfUltimateDestiny who would win in a fight?]]
* ThisTroper has seen umptynine ''LifeOnMars'' fics that reveal that Sam Tyler is... in fact, [[DoctorWho the Master.]]
** This troper isn't sure whether or not that's more ridiculous than the idea of Ianto being the Master's son...
* A target of frequent fic speculation, though little actual writing in this troper's experience, is voice actress KotonoMitsuishi, whose incredibly broad portfolio of roles offers a vast range of possibilities: imagine, if you will, an intersection of any two or more of [[NeonGenesisEvangelion Misato Katsuragi]], [[DragonHalf Mink]], [[SailorMoon Usagi Tsukino]], [[ExcelSaga Excel Excel]], [[DeadOrAlive Christie]], [[GundamSeedDestiny Murrue Ramius]], [[KaleidoStar Cathy Taymor]], [[FullMetalAlchemist Gracia Hughes]], [[FruitsBasket Kagura Soma]], [[{{Noir}} Mireille Bouquet]], [[RevolutionaryGirlUtena Juri Arisugawa]], [[OruchubanEbichu Ebichu]] and [[TetsuwanBirdy Birdy Cephon Altirra]], just to name a few.
* The OfficialFanfictionUniversity of Middle-Earth contained a chapter where the "teachers" (read: cast of ''LordOfTheRings'') were all replaced by other characters played by the same actors.
* ''{{Stargate SG-1}}''. ''{{MacGyver}}''. There are too many of these fics, far too many [=JackGuyvers=].
* ''BuffyTheVampireSlayer''/''{{Firefly}}'' crossovers have at least two 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.
** There is at least one fic out there which hits ''both'' of these, and ends with Wash plaintively asking when they'll find the world where ''he's'' worshipped as a god.
*** [[{{Dollhouse}} Ta-daaaaaa!]]
** Not to mention that there is -- as far as [[{{Turtleducks}} this troper]] can remember -- at least one ''{{Firefly}}''/''DrHorriblesSingAlongBlog'' crossover featuring Captain Hammer and Mal Reynolds meeting up.
* This troper has recently watched ''Robin Hood: Prince of Thieves'' after having not seen it in years and was struck to realize that the Sheriff of Nottingham bore a striking resemblance to [[HarryPotter Severus Snape]]. Mostly the similarity in the characters is in the accent, but it was very interesting to note one of the first scenes in the movie, where Sheriff [=NottingSnape=] and his allies are dressed in white robes and masks reminiscent of Death Eaters.
** [[TheHitchhikersGuideToTheGalaxy The Half-Blood Prince will have you know he's feeling very depressed.]] [[DieHard And that Dumbledore won't be joining us for the rest of his life.]]
* Philip Akin appeared as a regular in the first season of ''[[Series.WarOfTheWorlds War of the Worlds]]'', Adrian Paul in the second. Since both actors also appeared in ''HighlanderTheSeries'', 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.
* ''{{Andromeda}}'' all but canonized the notion that Dylan Hunt is the reincarnation of [[HerculesTheLegendaryJournies Hercules]]. This Troper first encountered the crossover in, of all things, a ''KnightRider'' fanfic.
* Although there are no known examples yet, somewhere there is a ''SwansCrossing''/''BuffyTheVampireSlayer'' fic just itching to be written.
* This troper saw a fanfic wherein [[FireflyTheSeries Jayne Cobb]] was [[{{Chuck}} John Casey]], (albeit via Buffy, Highlander, and Supernatural).
* [[http://robkelk.ottawa-anime.org/temp/1205104520634.jpg This.]]
* Masi Oka portrayed lab-guy Franklin on ''{{Scrubs}}'' and now portrays Hiro Nakamura on ''Series/{{Heroes}}'', a fact picked up on in one or two fanworks.
* ''BuffyTheVampireSlayer'' and ''VeronicaMars'' 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 [[{{X-Men}} Iceman]] and Mac dated [[{{Smallville}} The Flash]]. It's superhero central, guys.
* This troper (DaibhidC)'s own "Life of Doctor" is a drabble in which Judy from ''The Second Coming'' mistakes the Ninth Doctor for Steve Baxter. Being a drabble, that's the entire plot.
** He is also one of two authors who are determinedly crossing over ''everything'' StephenFry has ever appeared in, via the Minister of Chance from the ''DoctorWho'' webdrama "Death Comes To Time" either meeting his counterparts or taking on their characteristics.
** Claude from ''Series/{{Heroes}}'' being secretly the [[DoctorWho Ninth Doctor]] is a particularly common one.
** Oh, then there ought to be some for ''Our Friends in the North''. Come on: the [[DoctorWho Ninth Doctor]] ''and'' JamesBond ?
* [[SuzumiyaHaruhi Haruhi Suzumiya]] meeting [[LuckyStar Konata Izumi]]. That is all.
** [[DeathNote Misa Amane]] is optional is the writer takes the subs side of the SubsVsDubs wars.
* Done in reverse, with the official ''{{X-Men}}''/''StarTrekTheNextGeneration'' crossover which featured Captain Picard meeting Professor Xavier, but came out before the first {{X-Men}} movie.
* One can only speculate at just what the hell happened to [[{{Firefly}} 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 [[BattlestarGalactica President of the Colonies]], pretty impressive feat.
* The ''HarryPotter'' and ''{{Twilight}}'' series both feature actor Robert Pattinson in their respective film adaptations, although it would take some rather violent [[HandWave Hand Waving]] to make Cedric Diggory and Edward Cullen the same person. This has mostly been fodder for the [=MuggleNet=] CaptionContest.
** That said, Cedric seems to have become a much more popular character in ''Harry Potter'' fanfiction since the first ''Twilight'' movie.
* The first "Echo is [[BuffyTheVampireSlayer 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.
* Aside from the above example of Xanatos and Riker, ''{{Gargoyles}}'' and ''StarTrekTheNextGeneration'' 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 seem to be quite a few ''{{Supernatural}}'' and ''DarkAngel'' 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 TwinThreesomeFantasy trope, it was probably inevitable. However, ''Dark Angel'' is set TwentyMinutesIntoTheFuture. 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 Dean is Alec post-finale.
* Interestingly subverted in a few ''{{House}}'' fanfics that somehow add [[JeevesAndWooster Jeeves to the mix instead of Bertie]]. Guess which one was played by HughLaurie?
** [[spoiler: If you guessed "Bertie," you're right.]]
* Played ''amazingly'' straight in [[http://community.livejournal.com/indeedsir/506390.html?thread=7493142#t7493142 Jeeves and the Evil Twin]], a ''JeevesAndWooster'' fanfic in which Jeeves comes home from vacation to find that Jack Dalliard (one of StephenFry's recurring characters in ''ABitOfFryAndLaurie'') has been not only impersonating him but abusing and raping Bertie.
* Bringing this trope to new extremes, there's ''[[http://www.amazon.com/Shatnerquake-Jeff-Burk/dp/1933929820 Shatnerquake]]'', where every single character that WilliamShatner has ever played comes along to wail on the man himself.
* ''BusouRenkin'' fanfic writers like to crossover with ''RurouniKenshin'', usually making Kazuki into Kenshin's son.
* A fanfic that crossed over ''{{JAG}}'' with ''ThePretender'' 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.
* There are fics out there about Legolas meeting [[PiratesOfTheCarribeans Will Turner]] and/or [[KingdomOfHeaven Balian]] and/or Orlando Bloom himself.
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