The Collaborative International Dictionary of English v.0.48:

Doublet \Doub"let\, n. [In sense 3, OF. doublet; in sense 4, F.
   doublet, dim. of double double. See Double, a.]
   1. Two of the same kind; a pair; a couple.
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   2. (Print.) A word or words unintentionally doubled or set up
      a second time.
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   3. A close-fitting garment for men, covering the body from
      the neck to the waist or a little below. It was worn in
      Western Europe from the 15th to the 17th century.
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   4. (Lapidary Work) A counterfeit gem, composed of two pieces
      of crystal, with a layer of color between them, and thus
      giving the appearance of a naturally colored gem. Also, a
      piece of paste or glass covered by a veneer of real stone.
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   5. (Opt.) An arrangement of two lenses for a microscope,
      designed to correct spherical aberration and chromatic
      dispersion, thus rendering the image of an object more
      clear and distinct. --W. H. Wollaston.
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   6. pl. (See No. 1.) Two dice, each of which, when thrown, has
      the same number of spots on the face lying uppermost; as,
      to throw doublets.
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   7. pl. [Cf. Pr. doblier, dobler draughtboard.] A game
      somewhat like backgammon. --Halliwell.
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   8. One of two or more words in the same language derived by
      different courses from the same original from; as, crypt
      and grot are doublets; also, guard and ward; yard and
      garden; abridge and abbreviate, etc.
      [1913 Webster]

WordNet (r) 3.0 (2006):

doublet
    n 1: a man's close-fitting jacket; worn during the Renaissance

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "doublet":
   both, brace, cognate, couple, couplet, derivation, derivative,
   distich, double harness, duad, duet, duo, dyad, eponym, etymon,
   match, mates, pair, primitive, root, set of two, span, team,
   the two, twain, two, twosome, yoke