tunnage vs tonnage duty

tunnage and tonnage duty both are nouns.

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

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

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

Other synonyms of tunnage include tonnage.

tunnage (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