WordNet (r) 3.0 (2006):

doppelganger
    n 1: a ghostly double of a living person that haunts its living
         counterpart

Moby Thesaurus II by Grady Ward, 1.0:

32 Moby Thesaurus words for "Doppelganger":
   actual thing, carbon copy, co-walker, copy, dead ringer, ditto,
   double, doubleganger, duplicate, equivalent, etheric double,
   exact counterpart, facsimile, fetch, homograph, homonym, homophone,
   idem, identical same, no other, none other, replica, selfsame,
   spit and image, spitting image, synonym, the same,
   the same difference, twin, very image, very same, wraith