stamp duty vs stamp tax

stamp duty and stamp tax both are nouns.

Word	Noun	Adjective	Verb	Adverb
stamp duty	Yes	No	No	No
stamp tax	Yes	No	No	No

As nouns, stamp duty and stamp tax are synonyms defined as:

stamp duty and stamp tax: a tax collected by requiring a stamp to be purchased and attached (usually on documents or publications)

stamp duty (noun)	stamp tax (noun)
a tax collected by requiring a stamp to be purchased and attached (usually on documents or publications)	a tax collected by requiring a stamp to be purchased and attached (usually on documents or publications)