pair vs duo

pair and duo both are nouns.

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

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

Other hyponyms of pair include couple, duet, twosome, yoke.

As nouns, pair and duo are synonyms defined as:

Other synonyms of pair include brace, couple, couplet, distich, duad, duet, dyad, span, twain, twosome, yoke.

pair (noun)	duo (noun)
two people considered as a unit	a musical composition for two performers
a set of two similar things considered as a unit	a pair who associate with one another
a poker hand with 2 cards of the same value	two performers or singers who perform together
two items of the same kind	two items of the same kind

pair (verb)	duo (verb)
engage in sexual intercourse
bring two objects, ideas, or people together
arrange in pairs
occur in pairs
form a pair or pairs