couplet vs doubleton

couplet and doubleton both are nouns.

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

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

couplet: two items of the same kind
doubleton: (bridge) a pair of playing cards that are the only cards in their suit in the hand dealt to a player

couplet (noun)	doubleton (noun)
a stanza consisting of two successive lines of verse; usually rhymed	(bridge) a pair of playing cards that are the only cards in their suit in the hand dealt to a player
two items of the same kind