Billing Error

In July MTN changed the data dates. Our services were terminated and we were double billed. I called the call centre and we were reconnected and the double billing was to accommodate the data dates. By October my statement was still showing one month in arrears. I called the centre again with ref 1- ********** 3606 for an accounts supervisor to call me to solve the issue. I then received an sms to say the issue was resolved but it wasn't when I called back they said that the ref was incorrectly resolved. When I explained that my contract was ending the 4 November I was assured that I would be billed prorate for the November data. I also called in at the Norwood branch on 9 November because the contract had gone over to prepaid and I was unable the call centre. I spoke to Theo and he called them on my behalf and assured me that the billing will be prorata, I was billed for the full amount in December and I went to Norwood yesterday and they call the centre and they told me the billing is correct. I received my statement and I see that the extra amount billed was from the double billing in July 2018 which has been in arrears this whole time and not cleared. I kept insisting that it is from the double billing in July and the lady has inssisted that I received the data but how can we have received the double data?