mid-15c., "having two parts, double," from Latin duplicatus, past participle of duplicare "to double," from duo "two" (see two) + plicare "to fold" see ply (v.1)). Meaning "exactly corresponding, that is an exact copy of" is from 1812.
1620s, "to double," from Latin duplicatus, past participle of duplicare (see duplicate (adj.)). Meaning "make an exact copy" is from 1640s (implied in duplicated). Related: Duplicating. The noun is first recorded 1530s.