duty vs tonnage duty

duty and tonnage duty both are nouns.

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

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

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

duty (noun)	tonnage duty (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