From Latin 'duplus' (twofold) — 'duo' (two) + '-plus' (fold), from PIE *dwoh1 (two).
Consisting of two equal, identical, or similar parts or things; twice as much or as many. As a verb, to make twice as great or as many.
From Old French doble, from Latin duplus (twofold, double), a compound of duo (two) and -plus (fold, multiplication), from PIE *-plo- (fold). The first element, duo, descends from PIE *dwóh₁ (two), one of the oldest and most stable numerals in the Indo-European family: it appears in Sanskrit as dvá, Greek as dýo, Latin as duo, Old English as twā, Gothic as twai. The second element, -plus, is related to Latin plēre (to fill, to fold