duel has 1 distinct synonym spread across 1 of its 3 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a prearranged fight with deadly weapons by two people (accompanied by seconds) in order to settle a quarrel over a point of honor
any struggle between two skillful opponents (individuals or groups)
fight a duel, as over one's honor or a woman
“In the 19th century, men often dueled over small matters”
Synonyms, antonyms and the broader and narrower relations for duel come from Princeton University’s WordNet. Full sources and licences.