VERB (3)

1. double in magnitude, extent, or intensity;
- Example: "The enemy redoubled their screaming on the radio"

2. double again;
- Example: "The noise doubled and redoubled"

3. make twice as great or intense;
- Example: "The screaming redoubled"

The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

         So they
         Doubly redoubled strokes upon the foe.   --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Redouble \Re*dou"ble\, v. i.
   To become greatly or repeatedly increased; to be multiplied;
   to be greatly augmented; as, the noise redoubles.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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"

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "redouble":
   accelerate, aggravate, beef up, blow up, come again, complicate,
   concentrate, condense, consolidate, copy, deepen, ditto,
   do a repeat, do again, do over, double, dupe, duplicate, echo,
   enhance, exacerbate, exaggerate, geminate, heat up, heighten,
   hop up, hot up, imitate, ingeminate, intensify, jazz up, key up,
   magnify, make complex, mount, multiply by two, parrot, plagiarize,
   quote, ramify, redo, reduplicate, reecho, regurgitate, reincarnate,
   reinforce, renew, repeat, replicate, reproduce, revive, rise,
   rouse, say again, sharpen, soup up, step up, strengthen, triple,
   twin, whet