tonnage duty vs tonnage

tonnage duty and tonnage both are nouns.

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

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

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

Other synonyms of tonnage duty include tunnage.

tonnage duty (noun)	tonnage (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