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The similarities and differences of twins has long held a fascination for writers. Perhaps the oldest depiction of this idea is the biblical story of Cain and Abel in which one brother is the jealous counterpart of the other. Dualism and the conflict between good and evil is an important element in religion and mythology.

The concept also has its roots in the notion of a doppelgänger, a ghostly double whose appearance is an ill omen. The evil twin expands on this by being concrete and interacting physically with the world.

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from http://en.wikipedia.org/wiki/Evil_twin

Edited to say that I just realised you got exactly the same reply to this question at Ask.FunTrivia.com, citing the same source! Just shows how many people now-a-days rely on Wikipedia as their primary source of information.