Duel:
Noun
a prearranged combat between two persons, fought with deadly weapons according to an accepted code of procedure, especially to settle a private quarrel.
any contest between two persons or parties.
verb (used with or without object), du·eled, du·el·ing or (especially British) du·elled, du·el·ling.
to fight in a duel.

Dual:
adjective
of, relating to, or noting two.
composed or consisting of two people, items, parts, etc., together; twofold; double:
dual ownership; dual controls on a plane.
having a twofold, or double, character or nature.
Grammar. being or pertaining to a member of the category of number, as in Old English, Old Russian, or Arabic, that denotes two of the things in question.
noun Grammar.
the dual number.
a form in the dual, as Old English git “you two,” as contrasted with ge “you” referring to three or more.