Mistake one cannot make twice (5,5)

'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?