When a Doppelganger emerges from the shadows and attacks the Original they've replicated that person's world is turned upside down, and they're forced to fight themselves for survival.
Then Somebody Else shows up and tries to tell the difference between the two, and the whole thing turns into a game of "Pick me! No, pick me!" until one gets dropped.
But, as this Maximumble comic shows, if the Somebody Else who shows up is a total bore then death by doppelganger it is!
-Via Geeks Are Sexy