Source : Webster's Revised Unabridged Dictionary (1913)

Double first \Dou"ble first`\ (Eng. Universities)
      (a) A degree of the first class both in classics and
          mathematics.
      (b) One who gains at examinations the highest honor both
          in the classics and the mathematics. --Beaconsfield.

Source : WordNet®

double first
     n : a first-class honours degree in two subjects