doppelganger

doppelganger \DOP-uhl-gang-uhr\, noun:
1. A ghostly double or counterpart of a living person.2. Alter ego; double.
To readers of science fiction, the idea of a single atom existing simultaneously in two states or places is reminiscent of the supernatural "doppelganger" -- a flesh-and-blood duplicate of one's self encountered while walking along a street.-- "Physicists Put Atom in Two Places at Once", New York Times, May 28, 1996
But my primary interest here is not the machinations of science itself but the fascinating life and times of its dark doppelganger, the mad scientist, in all his overreaching glory.-- David J. Skal, Screams of Reason
Doppelganger is from the German doppel, "double" + Gänger, "goer."