The Collaborative International Dictionary of English v.0.48:

Dueler \Du"el*er\, n.
   One who engages in a duel. [R.] [Written also dueller.]
   --South.
   [1913 Webster]

WordNet (r) 3.0 (2006):

dueller
    n 1: a person who fights duels [syn: dueler, dueller,
         duelist, duellist]