If the overpayment is from a scheduled payment, or if there is a visible Bankgiro on the transaction, the money will be refunded automatically to the same account and will be visible within the next business day.
If you have made the payment in another way, the money will be refunded as a cheque.
There is always a new OCR number on every invoice. Make sure that you don't use a previous OCR number to avoid making an overpayment.