Impersonating the Evil Twin

The situation where Evil Twin is reversed—the Evil Twin isn't pretending to be the character, but instead the character is pretending to be the Evil Twin. Never mind that the original character hasn't studied the Evil Twin's typical mannerisms, knows nothing of his background and may not have his abilities. Never mind that the Evil Twin may be wearing a mask or equivalent which the real character can't take off since it's his real face. They look alike, so it has to work, right? In fact, it works more often than it should, often stopped only by gross problems (such as the real Evil Twin walking through the door), not by the dozens of flaws in the impersonation that nobody notices. Frequently it works because the Evil Twin's Mooks are too scared of him to question his changes in behavior.

Compare with Hugh Mann. Sometimes exposed by a Bluff the Impostor maneuver.

Anime & Manga

 * The Decepticon toy-only character Sky Shadow in the Transformers Cybertron has the same body mold as the Autobot Jetfire, which allows him to impersonate Jetfire as a spy. However, he actually is Jetfire, impersonating an impersonator in order to spy on the Decepticons. Confused yet?
 * In the Reigai arc in Bleach, disguises  as  reigai impostor. Using fake contact lenses.

Comics -- Books

 * The original Fantastic Four Skrulls story has the FF pretend to be the Skrull spies. They get away with it, too, since apparently the Skrulls don't ask them to speak Skrullian, let alone to change back into Skrull form. They also convince the Skrulls that Earth has amazing defences by showing them a science-fiction comic book. 1960s Skrulls clearly weren't very bright.
 * In one storyline of Teen Titans, the villain (mentally disturbed former Titan Jericho) can jump into people's bodies, and was last seen occupying Cyborg, so to draw out another hero (the Vigilante) who is also looking for him, Cyborg pretends to still be possessed and fights his teammates... only to get the robot half of his head shot out for his trouble when the Vigilante shows up. Sometimes this works too well...

Fan Works

 * Played with in With Strings Attached (naturally!). During the New Zork chapter, the real Ringo is automatically mistaken for “Beagle Ringo” when he accidentally teleports into the Plaza Hotel, even though he has a beard (everyone assumes he had it magically grown on during a drunken bender). Given that his presence makes some trouble for “Beagle Ringo,” this sort of makes him the Evil Twin through no fault of his own. (It also makes his beard a Beard of Evil.)

Films -- Live-Action

 * Pretty much all of the movie Face Off is a twist on this, with both hero and villain assuming each other's identities for different reasons (Archer to stop the bad guy's evil plot and Troy to mess with Archer's life royally). This is made easier through a process that allows them to switch faces, with body differences being taken care of as well. Archer has spent so much time pursuing Troy, that he knows everything about him and as such can properly imitate him. Troy on the other hand acts nothing like Archer, but as Archer was not liked by his coworkers or family they all like him more.
 * This was the ultimate twist in the 1993 film version of The Man in the Iron Mask.
 * The Avengers 1998. Mrs. Peel gets into Sir August's underground base by posing as her Evil Twin clone (she wears the clone's black leather jumpsuit and gives a password). Sir August thought that the clone survived the balloon crash and tricked Steed into thinking that it was the real Mrs. Peel. He realizes she isn't the clone (and sends Bailey after her) when he detects her going after his weather control machine. Note that none of this was stated in the movie, the audience had to figure it out for themselves.
 * Tom Lincoln in The Island isn't necessarily evil. He's just a rich playboy whose philandering ways have resulted in him having a few months left to live. But when his "insurance policy", i.e. his clone Lincoln Six Echo shows up at his home, he appears to be glad to find out about it. He then immediatley calls the cloning company and tells them about the clone. Later on, Lincoln Six Echo (conveniently wearing Tom's clothes) manages to slip his bracelet on Tom's arm, resulting in the latter's death. He further continues to imitate Tom by adopting his Scottish accent after hearing is for about an hour. Laurent, a trained mercenary, fails to detect any deception, even though the clone slipped the bracelet on Tom's arm in while Laurent was looking at them. Only later, while performing an autopsy on Tom's body, the doctor realizes that the bracelet is on the wrong arm. Meanwhile, the clone has infiltrated the cloning company as Tom (supposedly there to make a new clone), still using a Scottish accent and behaving like a rich playboy. Oh, and he says "shite" at one point, even though Tom never said the word in his presense.
 * Of course, one of the subplots in the film is that, so this is partially justified.

Gamebooks

 * In book 19 of Lone Wolf, the hero kills his Evil Twin, Wolf's Bane, and impersonates him. He successfully infiltrates the enemy base, and Wolf's Bane's superiors are fooled because Lone Wolf's aura was altered to match his. He is outed when he is forced to help Alyss, who has snuck in as well and gets spotted.

Literature
"Guard: But... Duke Slorpo hates pickles... Neal: Really? What kind of a nut is he? I mean, am I?"
 * Played both ways and then subverted multiple times in thriller The Third Twin by Ken Follett. Main character, plucky academic underdog, has uncovered evidence that eight identical twins (including her boyfriend) were implanted secretly (and nonconsensually) in separate mothers during the early days of in vitro fertilization; boyfriend's Evil Counterpart and titular Third Twin, who was raised by the Corrupt Corporate Executive who ran the experiment, spies on her on his father's behalf. Evil Counterpart's cover is blown when he shows a behavioral tic she saw his dad use, and then captured when Boyfriend intervenes. Boyfriend then impersonates Evil Counterpart—and succeeds for nearly 12 hours—only to have his cover blown when he fails to show a behavioral tic (a family in-joke).
 * In Keith Laumer's classic Imperium, a man is recruited by an interdimensional empire that occupies alternate Earths. His mission is to replace the overlord of one of these realities, who is his alternate self. The ruse works perfectly until the first time he confronts one of the overlord's inner circle—who, upon seeing him, immediately orders his arrest.
 * Done inadvertently in Marion Zimmer Bradley's The Door Through Space; the hero and his antagonist both fit the same general description, and when the hero realizes that the bad guys have mistaken him for the other guy he proceeds to take advantage as best he can.
 * Similarly, in the Animorphs story The Andalite Chronicles, Elfangor impersonates Visser Three shortly after the latter managed to take control of Alloran's body. The thing is, the Visser's subordinates only know that he's supposed to have an Andalite host now, but doesn't know which Andalite, so they believe Elfangor and obey his order... to attack the real Visser Three while Elfangor escapes. Oops.
 * Kim Kinnison does something very similar to this in Second Stage Lensmen. While not actually standing in for an evil version of himself, the position he rises to within the enemy organisation corresponds almost exactly to the one he occupies in the Galactic Patrol's hierarchy (one below the actual leadership but with major input into strategic decision making), and the mental discipline needed to stay in character matches this trope very well. Unfortunately the one being who has any suspicion of him and can call him on it does, and at the most critical point in the plan. The fact that Kinnison pretty much knows this is coming doesn't make what follows any easier for him.
 * An example from the children's book series Secrets of Droon. The Power Trio is exploring a lost city and find out that two of them are doubles of the Duke and his sister. The kid who looks like the Duke tries to get the guards to go away by ordering them to go make him a burger... but he asks for extra pickles, leading to this exchange:


 * Subverted in Ranger's Apprentice with . He has to impersonate his twin brother, but actually knows what his evil twin is like. It helps that his twin hardly ever appeared in public, so the impersonation was fairly easy to pull off. Those who had met his twin were surprised, and suspected something, but, not being aware that he had a twin in the first place, they weren't sure what to think.

Live-Action TV

 * Doctor Who
 * Subverted in The Chase. The Daleks have made a robot double of the Doctor, which he pulled the plug on. The travellers discuss having the Doctor pretending to be the robot, and conclude that it's an absurd idea because the Daleks should know that their own robot isn't working. The Doctor sneaks off in the middle of the conversation to do it anyway... and is caught in around five seconds.
 * In The Enemy of the World, the Doctor visits Earth during the year 2030, and is discovered to bear a shocking resemblance to a South American dictator named Salamander. The "good guys" ask him to impersonate Salamander and infiltrate his organization. At the end of the story, with his evil plans in ruins, Salamander makes an unsuccessful attempt to escape in the TARDIS by impersonating the Doctor.
 * Done also in The Android Invasion when the Doctor pretends to be his android double and successfully fools the other androids. He also succeeds in reprogramming his double to assist him, a fact which is only revealed after the double has done a Heroic Sacrifice in front of his horrified companion.
 * Done by the Doctor in the story Meglos; discovered only when Meglos is shot in the belief that he's the Doctor and replies, "satisfied?"
 * In Buffy the Vampire Slayer episode "Dopplegangland," Willow pretends to be evil vampire Willow. She gets caught due to being insufficiently evil, which one supposes you could call an error in the impersonation. However, it initially works well, and has an effect on helping the Scoobies' strategy, because the Mirror Universe Evil Twin's mooks don't know her much either and also are scared of her.
 * On Knight Rider, Michael Knight impersonates his evil twin, Garth, in "Goliath". Revealed when his Beard of Evil is torn off.
 * Hercules: The Legendary Journeys: Done when Hercules ends up in a Mirror Universe. In this case, though, he is undone by a flaw in his impersonation—specifically, by an offer of his "favorite" food, which his Evil Twin actually hated.
 * Star Trek
 * The only really good example of this in canon Star Trek is the original Mirror Universe episode "Mirror, Mirror". In Deep Space Nine Mirror Universe episodes, counterpart visitors are either known to be counterparts or were replacing good guys, and in Enterprise Mirror Universe episodes, there ARE no counterparts. Non-canon novels, comics, and fanfics still feature this trope, though.
 * In encounters with the Borg, Jean-Luc Picard occasionally imitates Locutus (his identity when he was assimilated), since Borg drones will still acknowledge him as such. He does this a couple of times on the show and extensively in the novel The Return.
 * Lampshaded at the end of "Mirror, Mirror", when Spock points out that Good Kirk and co., being civilized, were successfully able to pretend to be savages, while their savage counterparts lacked the self-control necessary to pull the reverse stunt.
 * In the Expanded Universe, Mirror Kira has now been replaced by a crazy Cardassian from this universe, who was surgically alternated years ago to resemble the Kira over here in part of a failed intelligent scheme. (Although this Cardassian was never seen in the series, an episode had Kira asserted to be her and surgically altered 'back'.) This attempt appears to have inexplicably worked, despite the replacement not being the correct species, from the correct universe, or even an alternate universe version of that person, and thus having obvious gaps in knowledge.
 * Sliders
 * Averted in the pilot, where Arturo does a fairly reasonable impersonation of his otherworld Communist general counterpart. The soldier lets him through, but still shows the wherewithal to call command and ask where the said general is.
 * Unfortunately, this trope was played all too straight as the series wore on.
 * On Charmed, "The Importance of Being Phoebe", the demon Kaia shapeshifted to appear like Phoebe, and Phoebe was able to impersonate Kaia in turn just by adopting some of her slutty mannerisms such as constantly playing with her hair.
 * The Prisoner episode "The Schizoid Man". After his double is killed, Number 6 decides to impersonate him in order to escape from The Village. He fails due to a Bluff the Impostor trick by Number 2.
 * An episode of Friends has Phoebe pretending to be her sister Ursula to break up with Joey.
 * Done in an episode of Stargate SG-1, when the team's counterparts from other realities started getting stuck in their world. One team was slightly eviler than the rest. Predictably, this situation resulted in a brain exploding amount of self-impersonations.
 * In one Hogan's Heroes episode, the Germans planned to fake the escape of a British officer, sending his double, who'd assassinate Churchill after arriving in England. The Heroes, of course, captured the German impostor, rescued the Brit, and let the Germans ship him safely off to Britain. When greeted by a German soldier, he covered up his inability to speak German by claiming he was sticking to the British disguise. "Speak in English, you fool; I'm Group Captain Roberts, RAF."
 * An episode of The Avengers, "Two's a Crowd" had foreign agents groom an unscrupulous male model who closely resembled Steed to murder and impersonate him. He didn't do a good enough job of the "murder" part, resulting in Steed infiltrating the spy ring. Essentially the same concept was reused in The New Avengers episode "Faces."
 * In The Legend of Dick and Dom, the Big Bad creates Evil Twin versions of the heroes; when this is discovered, Prince Dick impersonates one of them to discover what they are up to. Notably, his fake Beard of Evil is slightly more convincing than the doppleganger's real one, probably because the Big Bad is not very competent.
 * This trope is retty much the entire plot of Ringer, although Bridget doesn't know that Siobhan is still alive Bridget gets away with it most of the time, although

Video Games

 * Weird example in Metal Gear Solid 2, as it was coincidental. Solid Snake dresses as a fictional Navy SEAL in order to get into an offshore cleaning facility without arousing suspicion. He encounters Raiden, who instantly recognises him as being a dead ringer for Liquid Snake, another survivor of the cloning project which Solid Snake was born from—because of this, he's unnaturally suspicious. It doesn't help that the disguise is paper-thin—the director stated he wanted to make the disguise as transparent as possible to see how many players he could manage to fool nonetheless. (Add to this a third clone claiming to be Solid Snake while not even bothering to dress up, and you've got yourself a headache.) The game also credits the voice actor whenever a new character is introduced, so it doesn't take a rocket surgeon to put two and two together.
 * At the beginning of Final Fantasy XII, Basch was imprisoned for regicide on Dalmasca's sovereign, Ashe's father, though
 * Mother 3:

Web Comics

 * This is toyed with in a storyline for Bob and George—when the president is kidnapped by ninjas, Mike finds the ninjas obeying him without reason or rhyme, letting the president go without a fight. Later on, it's discovered that Mike's non-alternate universe variant (as Mike came from an alternate universe with Mynd) is their real boss. Well, was.
 * Inverted in Order of the Stick: Elan manages to (finally) use his twin situation to his advantage when he tricks a prisoner of into attacking Nale by making said prisoner believe Nale is him.

Western Animation

 * Played with in the Justice League Unlimited episode "The Great Brain Robbery". Flash's mind is stuck in Lex Luthor's body, and Flash's impersonation of Luthor largely consists of him ham-handedly talking about how "evil" all his actions are. He is only outed after Luthor (in Flash's body) exposes him. A few people do catch on to the ruse, but don't reveal it for their own reasons:
 * Tala prefers the gentler Flash as her lover. (So much for Hawkgirl's "fastest man on Earth" insult.)
 * Gorilla Grodd hates the real Luthor and is fine letting the ruse fall apart on its own.
 * Bizarro is... well, Bizarro, and unable to articulate his insight. "Am you Bizarro's mommy?"
 * To further confuse the point, the Flash really is Lex Luthor (on Smallville). That is to say, Michael Rosenbaum, who plays Luthor in Smallville, is also the voice of the Flash.
 * It's averted in the same episode by the fact that Dr. Fate, whose magic was involved in the mind switch, immediately points out that Luthor's mind has ended up in Flash's body. Luthor, for his part, doesn't even try to act like the Flash. And when he takes of his mask, he has "no idea who he is."
 * Also from Justice League, the League members are sent to a Alternate Universe where their counterparts, known as the Justice Lords, eventually went rogue and took the Well-Intentioned Extremist route. There's a point when the League members (excluding Batman) try to enter the alternate universe Arkham by pretending to be their counterparts, which they fail to pull off due to not following the secret password procedure correctly.
 * The trope is then subverted when the league are surrounded by the police; Lord Batman shows up and calls them off, escorting the League to safety. Superman at first thinks it's really their own Batman obeying this trope, but it turns out that it really is Lord Batman.
 * Scarlett from G.I. Joe is a Master of Disguise, but was typically horrible at actually impersonating somebody. In one episode, she attempts to impersonate an evil scientist's wife/partner. The trope is subverted in that she is immediately spotted because she did not greet "her" husband by insulting him. Typically Scarlett will get caught one way or another. Her evil counterpart -- The Baroness—and shapeshifter/illusionist Zartan are much more competent.
 * Darkwing Duck
 * Darkwing Duck tries to pull this off once. It actually works perfectly, but The Fearsome Five had just decided to betray Negaduck....
 * There's also an odd case where he actually turns himself into Negaduck in order to find the real Negaduck's hideout.
 * The Powerpuff Girls episode "Bubble Boy" has Bubbles impersonating her Spear Counterpart Boomer. However, she actually does have trouble with Boomers mannerisms (which apparently includes eating cockroaches) and Brick is almost immediately suspicious of her. And of course, there's still the question of how she manages to look exactly like Boomer just by putting down her hair...