The Collaborative International Dictionary of English v.0.48:

duplication \du`pli*ca"tion\, n. [L. duplicatio: cf. F.
   duplication.]
   1. The act of duplicating, or the state of being duplicated;
      a doubling; a folding over; a fold.
      [1913 Webster]

   2. (Biol.) The act or process of dividing by natural growth
      or spontaneous action; as, the duplication of cartilage
      cells. --Carpenter.
      [1913 Webster]

   duplication of the cube (Math.), the operation of finding a
      cube having a volume which is double that of a given cube.
      [1913 Webster]

WordNet (r) 3.0 (2006):

duplication
    n 1: a copy that corresponds to an original exactly; "he made a
         duplicate for the files" [syn: duplicate, duplication]
    2: the act of copying or making a duplicate (or duplicates) of
       something; "this kind of duplication is wasteful" [syn:
       duplication, gemination]

Moby Thesaurus II by Grady Ward, 1.0:

123 Moby Thesaurus words for "duplication":
   Janus, ambiguity, ambivalence, bedizenment, biformity, bifurcation,
   burlesque, clone, conjugation, copy, counterpart, dichotomy, ditto,
   double, doubleness, doublethink, doubling, dualism, duality, dummy,
   dupe, duplexity, duplicate, duplication of effort, duplicity, echo,
   embellishment, equivocality, expletive, extravagance, facsimile,
   fat, featherbedding, filling, frill, frills, frippery, gingerbread,
   halving, hectography, imitation, irony, knockoff, luxury,
   mimeography, mock-up, model, needlessness, ornamentation,
   overadornment, overlap, padding, pairing, palingenesis, paraphrase,
   parody, payroll padding, plagiarism, pleonasm, polarity, prolixity,
   quadruplicate, quotation, re-creation, re-formation, reappearance,
   rebirth, rebuilding, reconstitution, reconstruction, recurrence,
   redesign, redoing, redoubling, redundance, redundancy,
   reduplication, reecho, reedition, reestablishment, refashioning,
   regeneration, regenesis, regurgitation, reincarnation,
   reinstitution, reissue, remaking, renascence, renewal, renovation,
   reoccurrence, reorganization, repetition, replica, replication,
   representation, reprinting, reproduction, reprography, reshaping,
   restoration, restructuring, resumption, resurrection, return,
   revision, revival, rubbing, superfluity, superfluousness,
   tautology, tracing, transcription, travesty, triplicate, twinning,
   two-facedness, twoness, unnecessariness, verbosity, version,
   xerography