The noun duty has 3 senses (first 3 from tagged texts)
1. (236) duty -- (work that you are obliged to perform for moral or legal reasons; "the duties of the job")
2. (89) duty, responsibility, obligation -- (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"; "every right implies a responsibility; every opportunity, an obligation; every possession, a duty"- John D.Rockefeller Jr)
3. (29) duty, tariff -- (a government tax on imports or exports; "they signed a treaty to lower duties on trade between their countries")