couplet vs duet

couplet and duet both are nouns.

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

As nouns, couplet and duet are synonyms defined as:

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

couplet (noun)	duet (noun)
a stanza consisting of two successive lines of verse; usually rhymed	(ballet) a dance for two people (usually a ballerina and a danseur noble)
two items of the same kind	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