customs vs customs duty

customs and customs duty both are nouns.

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

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

Other synonyms of customs include custom, impost.

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