"twofold nature, state of being two or divided in two," late 14c., from Late Latin dualitas, from Latin dualis "that contains two; the dual number, duality," from duo "two" (from PIE root *dwo- "two").
It is the hypothetical source of/evidence for its existence is provided by: Sanskrit dvau, Avestan dva, Greek duo, Latin duo, Old Welsh dou, Lithuanian dvi, Old Church Slavonic duva, Old English twa, twegen, German zwei, Gothic twai "two;" first element in Hittite ta-ugash "two years old."