Happened to me once in BKK, hotel charged me twice for the room. I noticed online already before checkout, but the manager at the hotel couldn't help, or just wouldn’t.
Next day I went to my local bank office in Pattaya, and they sorted it in a few minutes, refund to my account after a day or two.
This was my Bangkok Bank Debit Card.
If you still have the cash receipt from the hotel, then take that and the printout of the transaction to your bank, they should be able to sort it out for you.