Crossword Genius

A question of double identity? (5,2,5)

Ross

I believe the answer is:

which is which

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

'double identity?' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'a question' is the wordplay.
I cannot really understand how this works, but
'question' could be 'which' (which is a question word) and 'which' is found within the answer.
This explanation may well be incorrect...

'of' is the link.

Can you help me to learn more?

I've seen this clue in The Telegraph and The Globe and Mail.
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