Differing in citadel — or not? (9)

I believe the answer is:

identical

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'or not?' is the definition.
I can't tell whether this definition defines the answer.

'differing in citadel' is the wordplay.
'differing' is an anagram indicator.
'in'+'citadel'='incitadel'
'incitadel' anagrammed gives 'IDENTICAL'.

Can you help me to learn more?

(Other definitions for identical that I've seen before include "Duplicate" , "Not any different" , "uniform" , "Impossible to tell apart" , "Indistinguishable" .)

I've seen this clue in The Times.

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