couplet vs duo

couplet and duo both are nouns.

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

As nouns, couplet and duo are synonyms defined as:

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

couplet (noun)	duo (noun)
a stanza consisting of two successive lines of verse; usually rhymed	a musical composition for two performers
two items of the same kind	a pair who associate with one another
	two performers or singers who perform together
	two items of the same kind