duet vs II

duet and II both are nouns.

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

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

duet: two items of the same kind
II: the cardinal number that is the sum of one and one or a numeral representing this number

Other hypernyms of duet include 2, deuce, two.

duet (noun)	II (noun)
(ballet) a dance for two people (usually a ballerina and a danseur noble)	the cardinal number that is the sum of one and one or a numeral representing this number
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