Twins (6)
(Other definitions for duo that I've seen before include "Two of a kind" , "Musical work for two" , "Twosome" , "They might play" , "rejected Arab's lute" .)
'twins' is the definition.
(twins are the symbol of the star sign Gemini)
This is the entire clue.
(Other definitions for gemini that I've seen before include "Castor and Pollux constellation" , "set of twinklers" , "Twins constellation" , "Twins star sign" , "space programme" .)