Joined in duet? (6)

I believe the answer is:

united

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'joined' is the definition.
(uniting is a kind of joining)

'in duet' is the wordplay.
'in duet' anagrams to 'UNITED'.
But, I am not clear how this is indicated.

Can you help me to learn more?

(Other definitions for united that I've seen before include "See 9" , "Joined into a single entity" , "See 18" , "Leeds, Sheffield or Manchester, perhaps" , "Untied, being allied" .)

I've seen this clue in The Globe and Mail and the Evening Standard.

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