duet vs opus

duet and opus both are nouns.

Word	Noun	Adjective	Verb	Adverb
duet	Yes	No	No	No
opus	Yes	No	No	No

As nouns, opus is a hypernym of duet; that is, opus is a word with a broader meaning than duet:

Other hypernyms of duet include composition, musical composition, piece, piece of music.

duet (noun)	opus (noun)
(ballet) a dance for two people (usually a ballerina and a danseur noble)	a musical work that has been created
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