dual / duel are similar-sounding terms with different meanings (referred to as homophones). To better understand the differences, see below for definitions, pronunciation guides, and example sentences using each term. 👇
dual: (adjective) a grammatical number category referring to two items or units as opposed to one item (singular) or more than two items (plural).
duel: (noun) a prearranged fight with deadly weapons by two people (accompanied by seconds) in order to settle a quarrel over a point of honor. (noun) any struggle between two skillful opponents (individuals or groups). (verb) fight a duel, as over one's honor or a woman.
dual: d·UW·ah·l
duel: d·UW·ah·l
(Examples are Wikipedia snippets under the CC ShareAlike 3.0 license. Definitions drawn from WordNet.)