tonnage vs duty

tonnage and duty both are nouns.

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

As nouns, duty is a hypernym of tonnage; that is, duty is a word with a broader meaning than tonnage:

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

Other hypernyms of tonnage include tariff.

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