At Oxford, the Thames is doubled (4)

I believe the answer is:

isis

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'at oxford the thames' is the definition.
I can't tell whether this defines the answer.

'is doubled' is the wordplay.
'doubled' shows that letters should be duplicated.
'is' duplicated is 'ISIS'.

Can you help me to learn more?

(Other definitions for isis that I've seen before include "Sister of Osiris" , "Egyptian goddess of nature" , "Egyptian goddess - Thames at Oxford" , "divinity" , "Exists twice for a goddess" .)

I've seen this clue in the Evening Standard.

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