In the US, they would reopen your account without telling you and start charging you fees and/or interest on the amount they overpaid. I know this because it happened to me last year when American Express mistakenly refunded a credit to me twice when closing an account (one check and one bank transfer). I never even cashed the check but apparently that didn’t make a difference to them.

I really hope this is true, always nice to hear about the robbing happening in the opposite direction.

I gave the money back when I realised what had happened, which was about 5 minutes later.

Do you mean you got your balance twice?

Yes. I gave it back.