duet vs pas de deux

duet and pas de deux both are nouns.

Word	Noun	Adjective	Verb	Adverb
duet	Yes	No	No	No
pas de deux	Yes	No	No	No

As nouns, duet and pas de deux are synonyms defined as:

duet and pas de deux: (ballet) a dance for two people (usually a ballerina and a danseur noble)

duet (noun)	pas de deux (noun)
(ballet) a dance for two people (usually a ballerina and a danseur noble)	(ballet) a dance for two people (usually a ballerina and a danseur noble)
a musical composition for two performers
a pair who associate with one another
two performers or singers who perform together
two items of the same kind