WordNet (r) 3.0 (2006):

dead ringer
    n 1: a person who is almost identical to another [syn: ringer,
         dead ringer, clone]

Moby Thesaurus II by Grady Ward, 1.0:

55 Moby Thesaurus words for "dead ringer":
   Doppelganger, actual thing, carbon copy, companion, copy, ditto,
   double, duplicate, effigy, equivalent, exact counterpart,
   exact likeness, facsimile, fellow, homograph, homonym, homophone,
   icon, idem, identical same, idol, image, likeness, living image,
   living picture, match, mate, miniature, mirroring, model, no other,
   none other, photograph, picture, portrait, reflection, replica,
   resemblance, rubbing, selfsame, semblance, shadow, similitude,
   simulacrum, spit and image, spitting image, synonym, the same,
   the same difference, trace, tracing, twin, very image,
   very picture, very same