II vs duo

II and duo both are nouns.

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

As nouns, duo is a hyponym of II; that is, duo 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
duo: two items of the same kind

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

II (noun)	duo (noun)
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