duo vs doubleton

duo and doubleton both are nouns.

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

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

duo: 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

duo (noun)	doubleton (noun)
a musical composition for two performers	(bridge) a pair of playing cards that are the only cards in their suit in the hand dealt to a player
a pair who associate with one another
two performers or singers who perform together
two items of the same kind