Customer Service refusing to refund Double Billing

I requested for settlement letter for my account ***. They sent it to me and I paid the amount on the letter on the 23rd which is the same date as the one on the letter, I sent proof of payment to the supplier same day. on the 28th they continued to deduct the debit after I have settled the account. When I called them to enquire they blatantly told me they will not be refunding the debit amount they deducted after I had fully paid the account- Please refund my money!!!