doubleton vs couplet

doubleton and couplet both are nouns.

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

As nouns, couplet is a hypernym of doubleton; that is, couplet is a word with a broader meaning than doubleton:

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

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

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