duty vs impost

duty and impost both are nouns.

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

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

duty (noun)	impost (noun)
the social force that binds you to the courses of action demanded by that force	the lowest stone in an arch -- from which it springs
work that you are obliged to perform for moral or legal reasons	money collected under a tariff
a government tax on imports or exports