The antonyms for the phrase "be duty to" are words that indicate a lack of obligation or responsibility. Some of these words include: optional, voluntary, unnecessary, dispensable, and elective. When something is optional, it is up to the individual to decide whether or not to do it. Voluntary tasks are those that are chosen freely and without coercion. If something is unnecessary, it is not required or needed. Dispensable tasks are those that can be done away with without any negative consequences. Finally, an elective task is one that can be chosen from a list of options. All of these words are antonyms of "be duty to," which implies a necessary obligation or responsibility.