duty vs custom

duty and custom both are nouns.

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

As nouns, custom is a hyponym of duty; that is, custom is a word with a more specific, narrower meaning than duty:

duty (noun)	custom (noun)
the social force that binds you to the courses of action demanded by that force	accepted or habitual practice
work that you are obliged to perform for moral or legal reasons	habitual patronage
a government tax on imports or exports	a specific practice of long standing
	money collected under a tariff

duty (adjective)	custom (adjective)
	made according to the specifications of an individual