Cancellation Of Contracts

I have cancelled 2 of my Cell C contracts due to very poor network and poor customer service. I have cancelled one of my contracts in April which i paid the settlement amount. This amount reflected on my May statement. On my June statement it shows that the contract is now paid off but they invoice me again for the same amount on my June statement. I have called them over 20 times to get this fixed but nothing happens. Last month one of the consultants told me that the mistake is on their side and they will have to do a reversal of that amount but to date nothing has happened. I have cancelled my second contract last month and paid the settlement amount and now they are taking out the mistake amount from this. I called them multiple times to explain again and again but the consultants don't understand what i am saying even through it shows clearly on my statements that they have double charged me. I need this to get sorted out ASAP. These last few months has been a nightmare having to deal with Cell C.