Source : Webster's Revised Unabridged Dictionary (1913)

Redouble \Re*dou"ble\ (r?*d?b"'l), v. t. [Pref. re- + double:
   cf. F. redoubler. Cf. {Reduplicate}.]
   To double again or repeatedly; to increase by continued or
   repeated additions; to augment greatly; to multiply.

         So they Doubly redoubled strokes upon the foe. --Shak.

Redouble \Re*dou"ble\, v. i.
   To become greatly or repeatedly increased; to be multiplied;
   to be greatly augmented; as, the noise redoubles.

Source : WordNet®

redouble
     v 1: double in magnitude, extent, or intensity; "The enemy
          redoubled their screaming on the radio"
     2: double again; "The noise doubled and redoubled"
     3: make twice as great or intense; "The screaming redoubled"