Sorry to pick on this thread but it's something that annoys me, duel and dual are different.
duel
du·el (dl, dy-)
n.
1. A prearranged, formal combat between two persons, usually fought to settle a point of honor.
2. A struggle for domination between two contending persons, groups, or ideas.
v. du·eled or du·elled, du·el·ing or du·el·ling, du·els
v.tr.
1. To engage (another) in or as if in formal combat.
2. To oppose actively and forcefully.
v.intr.
To engage in or as if in formal combat.
dual
du·al (dl, dy-)
adj.
1. Composed of two usually like or complementary parts; double: dual controls for pilot and copilot; a car with dual exhaust pipes.
2. Having a double character or purpose: a belief in the dual nature of reality.
3. Grammar Of, relating to, or being a number category that indicates two persons or things, as in Greek, Sanskrit, and Old English.
n. Grammar
1. The dual number.
2. An inflected form of a noun, adjective, pronoun, or verb used with two items or people.