The words dual, duel sound the same but have different meanings and spellings. Why do dual, duel sound the same even though they are completely different words?
The answer is simple: dual, duel are homophones of the English language.
Composed of two usually like or complementary parts; double: dual controls for pilot and copilot; a car with dual exhaust pipes.
Having a double character or purpose: a belief in the dual nature of reality.
Grammar Of, relating to, or being a number category that indicates two persons or things, as in Greek, Sanskrit, and Old English.
Grammar The dual number.
A prearranged, formal combat between two persons, usually fought to settle a point of honor.
A struggle for domination between two contending persons, groups, or ideas.
To engage (another) in or as if in formal combat.
To oppose actively and forcefully.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.