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]