An Evil Twin is a popular concept and stock character found in almost all media imaginable, before that the concept could be found in fairy tales, folklore and even some religious works - though like many archetypes the use of "Evil Twins" has been criticized by some who see it as an overused topic, despite this it remains one of the most popular concepts in fiction.
Despite the name not all Evil Twins have to be literal twins, in fact most are not related to the main character (or are "corrupted" versions of the main character, sometimes from a "Mirror Universe").
Although normally existing as two separate individuals there is a disturbing sub-genre of the Evil Twin concept known as the "Parasitic Twin" and based very loosely on the deformity of the same name, in these stories the "Evil Twin" exists as a parasitic being within the protagonist and may either work against them or seek to corrupt them, this often merges with body horror and psychological horror.
Media Dealing With "Evil Twins"
- Covetous (in this video-game you play the role of a parasitic twin)
Famous "Evil Twins"
- Spock (Mirror Universe) - the iconic "evil twin", an alternate-reality Spock and the one that (infamously) began the trend of "evil twins" having pointy beards to signal they are antagonists.