Twins in tennis tournament (7)

I believe the answer is:

doubles

I'm a little stuck... Click here to teach me more about this clue!

'twins in tennis' is the definition.
'doubles' can be an answer for 'tennis' (doubles is a kind of tennis). I am unsure of the 'twins in' bit.

I can't explain the remainder of the clue.

Can you help me to learn more?

(Other definitions for doubles that I've seen before include "Tennis match for four players" , "sort of game" , "Like Carton and Darnay" , "Made twice as much" , "match in court" .)

I've seen this clue in The Sun.

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