I already refunded it yesterday. (OP's topic was split from elsewhere: he posted in a bunch of random topics while also sending me 3 PMs in rapid succession...)
He paid to his account-unique address, so verification wasn't a problem. If he'd paid to a different address, then I would require a signature.
This is very rare to happen
It's less rare than you might think; it happens once or twice a year, sometimes with very large amounts...