Charged double for a settled account

After cancelling my account with Telkom mobile and settling the outstanding balance at the end of last month. Now they send me an invoice for double my monthly payment!!! How can they charge me for an account that was cancelled and settled a month ago?? I even went in store to cancel the contract and was informed that the payment that would come off my bank account would be my last payment. Alas this is not the case. Their customer service is absolutely shocking and their products useless hence me leaving them. It's truly *******. Don't not ever use their services. I will probably not receive any feed back from them as they clearly don't care about customer service.