Evil Twin

:You may be looking for Evil Twin Corp. or Evil Twin Press.
While not necessarily related, an evil twin is a physical copy of the protagonist but with a radically inverted morality. The evil twin often functions to cause havoc in the protagonist's life, as most people are unable to tell them apart. 
A typical plot resolution will involve another character having to choose between the two, often at gunpoint or at the end of some other weapon, with "twins" trying to convince others of their authenticity. The good twin is often revealed to be the one who is more self-sacrificing or is the one with a more intimate knowledge of the relationship.
The evil twin trope differs from Mirror Universe fiction in that only one character has a counterpart.
==Physical Evil Twin Clues==
* Has a beard (see TV Tropes) - Spock's Goatee in the Mirror Universe episode of Star Trek: The Original Series is a famous example, despite the fact that he isn't evil
* Other hair changes (see TV Tropes) - Teal'c is seen with an unfamiliar pony tail, denoting him the evil version in a Stargate SG-1 episode
* Difference in speech - Data's evil brother Lore could use contractions whereas Data could not, but also just had a more dynamic intonation; Vampire Willow on Buffy the Vampire Slayer had a sultry, laconic voice and was so identified with one particular line of dialogue — "Bored now" — that it was later used to indicate when the "real" Willow had turned evil.
* Psychological differences - The "evil" version of Xander Harris in the episode The Replacement seemed more confident and successful than the "real" Xander. It later emerged that both were much the same, except that the "evil" Xander had had better luck on the day they were split apart.
==Canon==
Warning - SPOILERS!
* As mentioned above, the Buffy episode The Replacement appears to introduce a second magically-created Xander Harris. In fact he has been split into two people by the spell, and both will die if either is killed. Themes mentioned but not explored in the action of the story include group sex (with Anya Jenkins) and psychological experimentation to compare their personalities.
* Most versions of Superman have been cloned at least once. The most notorious variant is Bizarro, an imperfect copy with similar but "reversed" powers and personality. Lois & Clark: The New Adventures of Superman had several clone episodes including the creation of another Superman who regarded the original as his deadly rival, another Lois Lane who wanted to kill the original and was briefly married to Clark Kent, and (strongly implied but never actually explicitly shown) at least one Lex Luthor clone who is not noticably less evil than the original.
* The TV version of Supergirl has encountered a Nazi doppleganger from another dimension. Evil versions of Green Arrow and other heroes also appeared in the story arc.
* An evil version of Superboy appeared in the 1978 Crisis on Infinite Earths and later comics series and murdered numerous superheroes.
* The main villain in the first season of The Flash (2014) was Doctor Harrison Wells - or rather, a doppleganger since the real Wells had in fact been murdered before the show began. Later non-evil versions of the character from other dimensions appeared.
* The Middleman episode The Palindrome Reversal Palindrome was largely set in a parallel world run by evil versions of series regulars. The titular Middleman is a burned-out slacker in this world.
==Fanon==
* The End of Daylight by KateCMT has Philip Corman's Evil Twin tormenting Hutch for his role in his brother's death.
*In the Starsky and Hutch story Dead Man's Vengeance by Karen B., Val Bellamy, Vic's twin has revenge in mind
* The Sentinel zines Masks and The Devil You Know by Susan L. Williams have an "evil twin" Jim brutally attack Blair as he tries to take the real Jim's place
* Blair's lookalike from another dimension arrives to cause havoc in Genetic Destiny by Datalore.
* In the zine Fate's Reflection, when someone comes through the Stargate wearing Jack's face, he finds himself and his team thrown into a war that could bear impossibly high costs for them all.
* Breaking the Silence by Morgana includes an evil twin who tries to murder Blair.
* Substitute from Galactica 1988: Ten Years Later - an evil twin story in which Starbuck is captured by the Cylons and replaced with a clone that acts in an exaggerated manner, causing everyone to think there is something seriously wrong with him.