duty vs tonnage

duty and tonnage both are nouns.

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

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

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

duty (noun)	tonnage (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