Doppelgänger
From Memory Alpha, the free Star Trek reference.
Doppelgänger is a word from the German language which means an exact duplicate of a living person. The term doppelganger has been used throughout history, most often when someone meets an identical of themselves who is not a biological twin.
In 2366, Jean-Luc Picard was replaced by an alien copy and, upon returning to the USS Enterprise-D after his ordeal, commented that he "hoped his doppelganger hadn't caused too much trouble." (TNG: "Allegiance")
