II vs duet

II and duet both are nouns.

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

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

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

Other hyponyms of II include craps, snake eyes, brace, couple, couplet, distich, duad, duo, dyad, pair, span, twain, twosome, yoke.

II (noun)	duet (noun)
the cardinal number that is the sum of one and one or a numeral representing this number	(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