Contract Cancellation Charged Twice

I cancelled 2 contracts from Cell C on 23 October 2015, and paid in full for both phones (R6,374 & R4,816). I got sms confirmation on 25th that both phones contracts were successfully cancelled. 25 November they debit R6,374 for the phone that is already on prepaid. I called them and they agree that they debited money that was already paid. After holding for 20 minutes on the phone, they said they will investigate and get back to me after 2 working days. My other legit debit orders will bounce because they bankrupted by account and are treating this matter lightly. BIGGEST INCONVENIENCE EVER.