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.
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   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]