customs duty vs customs

customs duty and customs both are nouns.

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

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

Other synonyms of customs duty include custom, impost.

customs duty (noun)	customs (noun)
money collected under a tariff	money collected under a tariff