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I then tell MtGox staff that transaction A didn't confirm. They check it and they see transaction A was rejected and will never confirm but what they don't realize is it was just changed to transaction B by me, so they resend the transaction.

Profit.
Can they not just go back to such conversations with people that claimed this, and then figure out who was scamming them? Is there some sort of plausible deniability here for the scammer in this method that I'm missing, or not?
They should know who got extra coins out, but if they cleared their account there isn't much to do. There is even possibility they didn't provide real ID...