I would like to say that sometimes it does slip out by accident (maybe because of going to the pub so much). I consider ONCE as correct though if I'm talking to a friend in NZ and they say 'one time' I wouldn't think that their grammar is incorrect or bad or whatever, nobody speaks perfect grammar all the time and I'm sure that this 'one time' instead of 'once' appears in some other countries too. BUT if you are going to do the exam, make sure you only write ONCE.
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