Crossword Genius

Mistake one cannot make twice (5,5)

Ross

I believe the answer is:

fatal error

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

'one cannot make twice' is the definition.
I know nothing about this answer so I can't judge whether this works.

'mistake' is the wordplay.
I cannot quite see how this works, but
'mistake' could be 'error' (synonyms) and 'error' is found within the answer.
The remaining letters 'fatal' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?

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