Mistake turning up in copy, twice (4)
I believe the answer is:
typo
'mistake' is the definition.
(typo is a kind of mistake)
'turning up in copy twice' is the wordplay.
'turning up' shows that the letters should be reversed in order.
'in' indicates the answer is hidden within the clue.
'opyt' is hidden within 'copy twice'.
'opyt' back-to-front is 'TYPO'.
(Other definitions for typo that I've seen before include "Printer's mistake" , "What's not right" , "Misprint" , "Finger problem" , "Mistake in printed matter, in short" .)