duty has 3 distinct synonyms spread across 2 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.
the social force that binds you to the courses of action demanded by that force
“we must instill a sense of duty in our children”
Synonyms, antonyms and the broader and narrower relations for duty come from Princeton University’s WordNet. Full sources and licences.
work that you are obliged to perform for moral or legal reasons
“the duties of the job”
a government tax on imports or exports
“they signed a treaty to lower duties on trade between their countries”