doppelganger (DOP-uhl-gang-er) noun
A ghostly counterpart or double of a living person.
[From German, literally a double goer.]
"The classic doppelganger experience is a common theme in fiction where the appearance of the double often announces the hero's death by suicide. Probably the most dramatic illustration is Edgar Allan Poe's William Wilson, who in an attempt to stab his double, kills himself."
-- Raj Persaud; How You Could Meet Yourself; The Daily Telegraph (London); July 19, 2000.