So I've got a cheque that I need to pay into my bank. I've learned that I can just take a photo of it and get it deposited rather have to post it. It occurred to me that in the olden days when one paid in a cheque to a bank they would keep it, but now I can just keep it. So what's to stop me taking it to the post office and depositing that same "used" cheque into another account? Not that I would of course, just curious.