The Collaborative International Dictionary of English v.0.48:

Duple \Du"ple\, a. [L. duplus. See Double.]
   Double.
   [1913 Webster]

   Duple ratio (Math.), that in which the antecedent term is
      double the consequent, as of 2 to 1, 8 to 4, etc.
      [1913 Webster]