outstanding balance

The cosmos hospital keep on sending invoices and demands to pay my account that was settled on the 03-08-2015. I just phoned their debt recovery division to asked them what is going on, and why am I getting these letters all the time in the mail, when I have already made my payment in full to them! This is really bad administration service on their side and they gave me the wrong account number and they gave the wrong reference number for my accident that I had. I have sent them proof of payment AGAIN on the 12 of September 2015 to their fax number *** at 01:01PM.The admin women sent me an sms with the amount of R720 and she gave me the bank account details of *** and Ref. number of 17494, the Ref. number they gave me today was ***7 and the amount is R783.02, I'm sorry but I will not pay anything else to them! they got their money and they must get people who can do admin work!!!!