A person who seems to be an exact copy of yourself. Doppelgangers usually cause trouble by acting as yourself and interacting with other people. It is said that if you come into contact with your Doppelganger that you will die instantly, leaving the Doppelganger to play out your own life.
Bob: Hey, Fred, I don't like you being around Sara anymore, she says that you creep her out.
Fred: What are you talking about, man, I haven't seen Sara for years, is she in town?!
Bob: Yeah, I guess it was just a Doppelganger.