The Collaborative International Dictionary of English v.0.48:

Double dealer \Dou"ble deal"er\, double-dealer \dou"ble-deal"er\
   One who practices double dealing; a deceitful, trickish
   person; a person who says one thing and does another.
   --L'Estrange.

   Syn: double-crosser, betrayer, traitor.
        [1913 Webster + WordNet 1.5] Double dealing

WordNet (r) 3.0 (2006):

double-dealer
    n 1: a person who says one thing and does another [syn: double-
         crosser, double-dealer, two-timer, betrayer,
         traitor]