musical group vs duet

musical group and duet both are nouns.

Word	Noun	Adjective	Verb	Adverb
musical group	Yes	No	No	No
duet	Yes	No	No	No

As nouns, duet is a hyponym of musical group; that is, duet is a word with a more specific, narrower meaning than musical group:

musical group (noun)	duet (noun)
an organization of musicians who perform together	(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