duty vs tunnage

duty and tunnage both are nouns.

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

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

duty: a government tax on imports or exports
tunnage: a tax imposed on ships that enter the US; based on the tonnage of the ship

duty (noun)	tunnage (noun)
the social force that binds you to the courses of action demanded by that force	a tax imposed on ships that enter the US; based on the tonnage of the ship
work that you are obliged to perform for moral or legal reasons
a government tax on imports or exports