The Collaborative International Dictionary of English v.0.48:

Again \A*gain"\ (?; 277), adv. [OE. agein, agayn, AS. ongegn,
   onge['a]n, against, again; on + ge['a]n, akin to Ger. gegewn
   against, Icel. gegn. Cf. Gainsay.]
   1. In return, back; as, bring us word again.
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   2. Another time; once more; anew.
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            If a man die, shall he live again?    --Job xiv. 14.
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   3. Once repeated; -- of quantity; as, as large again, half as
      much again.
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   4. In any other place. [Archaic] --Bacon.
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   5. On the other hand. "The one is my sovereign . . . the
      other again is my kinsman." --Shak.
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   6. Moreover; besides; further.
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            Again, it is of great consequence to avoid, etc.
                                                  --Herschel.
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   Again and again, more than once; often; repeatedly.

   Now and again, now and then; occasionally.

   To and again, to and fro. [Obs.] --De Foe.
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   Note: Again was formerly used in many verbal combinations,
         as, again-witness, to witness against; again-ride, to
         ride against; again-come, to come against, to
         encounter; again-bring, to bring back, etc.
         [1913 Webster] Again

WordNet (r) 3.0 (2006):

again and again
    adv 1: repeatedly; "the unknown word turned up over and over
           again in the text" [syn: over and over, again and
           again, over and over again, time and again, time
           and time again]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "again and again":
   ad infinitum, at a stretch, ceaselessly, commonly, connectedly,
   constantly, continually, continuously, cumulatively, cyclically,
   day after day, day by day, endlessly, frequently, habitually,
   in many instances, incessantly, interminably, many a time,
   many times, many times over, monotonously, most often, much,
   not infrequently, not seldom, oft, often, often enough, oftentimes,
   ofttimes, on a stretch, on and on, on end, ordinarily,
   over and over, perennially, recurrently, repeatedly, repetitively,
   round the clock, routinely, several times, time after time,
   time and again, times without number, together, unbrokenly,
   unceasingly, unintermittently, uninterruptedly, unrelievedly,
   unseldom, usually, whenever you wish, without a break,
   without cease, without stopping, year after year