Doppelganger is from the German for double-goer. Do you ever worry that there is another version of you out there somewhere?
Borges writes of the Double:
Suggested or inspired by mirrors, the surface of still water, and twins, the concept of the Double is common to many lands. It seems likely that statements such as Pythagoras' "A friend is another myself" and Plato's "Know thyself" were inspired by it. In Germany it is called the Doppelganger, in Scotland, the fetch, because it comes to fetch men to their death. Meeting oneself was, therefore, most ominous; the tragic ballad Ticonderoga by Robert Louis Stevenson recounts a legend on this theme. We might also recall that strange painting by Rosetti called "How They Met Themselves" -- two lovers meet themselves at dusk in a forest.
Here is SNL's take on the phenomenon of evil twins.