The Collaborative International Dictionary of English v.0.48:

Doubler \Dou"bler\, n.
   1. One who, or that which, doubles.
      [1913 Webster]

   2. (Elec.) An instrument for augmenting a very small quantity
      of electricity, so as to render it manifest by sparks or
      the electroscope.
      [1913 Webster]

   3. A part of a distilling apparatus for intercepting the
      heavier fractions and returning them to be redistilled.
      [Webster 1913 Suppl.]

   4. (Calico Printing) A blanket or felt placed between the
      fabric and the printing table or cylinder.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

doubler
    n 1: an electronic device that doubles the voltage or the
         frequency of an input signal