The Collaborative International Dictionary of English v.0.48:

Duplicate \Du"pli*cate\, n.
   1. That which exactly resembles or corresponds to something
      else; another, correspondent to the first; hence, a copy;
      a transcript; a counterpart.
      [1913 Webster]

            I send a duplicate both of it and my last dispatch.
                                                  -- Sir W.
                                                  Temple.
      [1913 Webster]

   2. (Law) An original instrument repeated; a document which is
      the same as another in all essential particulars, and
      differing from a mere copy in having all the validity of
      an original. --Burrill.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Duplicate \Du"pli*cate\, v. t. [imp. & p. p. Duplicated; p.
   pr. & vb. n. Duplicating.]
   1. To double; to fold; to render double.
      [1913 Webster]

   2. To make a duplicate of (something); to make a copy or
      transcript of. --Glanvill.
      [1913 Webster]

   3. (Biol.) To divide into two by natural growth or
      spontaneous action; as, infusoria duplicate themselves.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

duplicate \du"pli*cate\, a. [L. duplicatus, p. p. of duplicare
   to double, fr. duplex double, twofold. See Duplex.]
   Double; twofold.
   [1913 Webster]

   Duplicate proportion or Duplicate ratio (Math.), the
      proportion or ratio of squares. Thus, in geometrical
      proportion, the first term to the third is said to be in a
      duplicate ratio of the first to the second, or as its
      square is to the square of the second. Thus, in 2, 4, 8,
      16, the ratio of 2 to 8 is a duplicate of that of 2 to 4,
      or as the square of 2 is to the square of 4.
      [1913 Webster]

WordNet (r) 3.0 (2006):

duplicate
    adj 1: identically copied from an original; "a duplicate key"
    2: being two identical [syn: duplicate, matching, twin(a),
       twinned]
    n 1: something additional of the same kind; "he always carried
         extras in case of an emergency" [syn: extra, duplicate]
    2: a copy that corresponds to an original exactly; "he made a
       duplicate for the files" [syn: duplicate, duplication]
    v 1: make or do or perform again; "He could never replicate his
         brilliant performance of the magic trick" [syn:
         duplicate, reduplicate, double, repeat,
         replicate]
    2: duplicate or match; "The polished surface twinned his face
       and chest in reverse" [syn: twin, duplicate, parallel]
    3: make a duplicate or duplicates of; "Could you please
       duplicate this letter for me?"
    4: increase twofold; "The population doubled within 50 years"
       [syn: double, duplicate]

Moby Thesaurus II by Grady Ward, 1.0:

189 Moby Thesaurus words for "duplicate":
   Doppelganger, Janus-like, Photostat, Xerox, Xerox copy,
   actual thing, alike, all one, all the same, ambidextrous,
   analogous, bifacial, bifold, biform, bilateral, binary, binate,
   biparous, bivalent, carbon, carbon copy, clone, come again,
   companion, comparable, conduplicate, conduplication,
   consubstantial, copy, copying, correspondent, corresponding,
   counterpart, dead ringer, disomatous, ditto, do a repeat, do again,
   do over, double, double-faced, doubling, dual, dupe, duple, duplex,
   duplication, echo, effigy, equal, equivalent, exact counterpart,
   exact likeness, exactly alike, facsimile, fellow, geminate,
   geminated, gemination, hectograph, homograph, homonym, homoousian,
   homophone, icon, idem, identic, identical, identical same, idol,
   image, imitate, imitation, indistinguishable, ingeminate,
   ingemination, iteration, just alike, like, likeness, living image,
   living picture, make over, manifold, match, matching, mate,
   microcopy, microfilm, mimeo, mimeograph, miniature, mirroring,
   model, multigraph, multiply by two, no other, none other,
   of a kind, of a piece, of a size, one, parallel, paralleling,
   parrot, photocopy, photograph, picture, plagiarize, portrait,
   quadruplicate, quote, re-create, re-form, rebuild, reconstitute,
   reconstruct, redesign, redo, redouble, reduplicate, reduplication,
   reecho, reestablish, refashion, reflection, refound, regenerate,
   regurgitate, reincarnate, reinstitute, reissue, reiteration,
   remake, renew, renovate, reorganize, repeat, repetition, replica,
   replicate, replication, representation, reprint, reproduce,
   reproduction, resemblance, reshape, restore, restructure,
   resurrect, revise, revive, rubbing, same, say again, second,
   secondary, selfsame, semblance, shadow, similitude, simulacrum,
   spit and image, spitting image, stat, synonym, tantamount,
   the same, the same difference, trace, tracing, transcribe,
   triplicate, twin, twinned, twinning, two-faced, two-level, two-ply,
   two-sided, two-story, twofold, undifferent, very image,
   very picture, very same, without difference, without distinction