The Collaborative International Dictionary of English v.0.48:

Twin \Twin\, v. i. [imp. & p. p. Twinned; p. pr. & vb. n.
   Twinning.]
   1. To bring forth twins. --Tusser.
      [1913 Webster]

   2. To be born at the same birth. --Shak.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Twinned \Twinned\, a. (Crystallog.)
   Composed of parts united according to a law of twinning. See
   Twin, n., 4.
   [1913 Webster]

WordNet (r) 3.0 (2006):

twinned
    adj 1: being two identical [syn: duplicate, matching,
           twin(a), twinned]