Spare lockers? (9,4)

I believe the answer is:

duplicate keys

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

'spare' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'lockers?' is the wordplay.
I cannot quite understand how this works, but
'lockers?' could be 'keys' (I've seen this before) and 'keys' is located in the answer.
The remaining letters 'duplicate' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?

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

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