legal duty vs duty

legal duty and duty both are nouns.

Word	Noun	Adjective	Verb	Adverb
legal duty	Yes	No	No	No
duty	Yes	No	No	No

As nouns, duty is a hypernym of legal duty; that is, duty is a word with a broader meaning than legal duty:

legal duty: acts which the law requires be done or forborne
duty: the social force that binds you to the courses of action demanded by that force

Other hypernyms of legal duty include obligation, responsibility.

legal duty (noun)	duty (noun)
acts which the law requires be done or forborne	the social force that binds you to the courses of action demanded by that force
	work that you are obliged to perform for moral or legal reasons
	a government tax on imports or exports