tonnage vs tonnage duty

tonnage and tonnage duty both are nouns.

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

As nouns, tonnage and tonnage duty are synonyms defined as:

tonnage and tonnage duty: a tax imposed on ships that enter the US; based on the tonnage of the ship

Other synonyms of tonnage include tunnage.

tonnage (noun)	tonnage duty (noun)
a tax imposed on ships that enter the US; based on the tonnage of the ship	a tax imposed on ships that enter the US; based on the tonnage of the ship