dop·pel·gäng·er or dop·pel·gang·er 
(dŏpəl-găng′ər, -gĕng′-)
(dŏpəl-găng′ər, -gĕng′-)n.
1. A ghostly double of a living person, especially one that haunts its fleshly counterpart.
2. Either of two people who physically resemble each other very much.
[German, a double : doppel, double (from French double; see DOUBLE) + Gänger, goer (from Gang, a going, from Middle High German ganc, from Old High German).]