redundancy (n.)

"superfluous, exceeding what is natural or necessary," c. 1600, from Latin redundantem (nominative redundans), present participle of redundare, literally "overflow, pour over; be over-full;" figuratively "be in excess," from re- "again" (see re-) + undare "rise in waves," from unda "a wave" (from PIE *unda-, nasalized form of root *wed- (1) "water; wet").

Also sometimes in 17c. in a more positive sense, "abounding to excess or fullness, exuberant, plentiful," e.g. in "Paradise Lost," though what he meant by it here is anyone's guess:

With burnished neck of verdant gold, erect

Amidst his circling spires that on the grass

Floated redundant.

 Of persons, in employment situations by 1928, chiefly British. Related: Redundantly. As a verb, redund has been tried at least once (1904); the etymological corresponding verb is the Frenchified redound.

word-forming element denoting quality or state, from Latin -antia, forming abstract nouns on past-participle adjectives in -antem, appearing in English mostly in words borrowed directly from Latin (those passing through French usually have -ance or -ence; see -ance). But English also has kept many pairs of words in -ance and -ancy (radiance/radiancy, etc.). Though typically one of the two forms has been more common, both were kept "as metrically useful or rhetorically effective" [Fowler, 1926].