tonnage duty vs tunnage

tonnage duty and tunnage both are nouns.

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

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

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

Other synonyms of tonnage duty include tonnage.

tonnage duty (noun)	tunnage (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