duo vs couplet

duo and couplet both are nouns.

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

As nouns, duo and couplet are synonyms defined as:

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

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