Billing discrepancy - no resolution from MTN

Mid May this year, I was contacted by MTN RewardsCo for an upgrade. They processed a new line instead, which I cancelled immediately the next day and had my upgrade processed correctly. I was then sent the device that was cancelled, on the same day my upgrade was delivered. I returned it with the DSV courier, and was guaranteed that I would not be charged for the returned device. In June, I was charged for the returned phone and modem as well as my upgrade, and this month July I am charged for it again. I have been unsuccessful in getting this sorted, even though the agents at customer care have confirmed that I only have 1 active contract with MTN and that I should not be billed for a contract that I do not have. The Billings dept has tried assisting in getting this resolved however I am being sent from Mondo to RewardsCo back to MTN. I've even made 2 *****ulent activity cases only to be told that they cannot help me. It is well over 2 months now, and I am getting nowhere with getting my money back nor getting my account with MTN corrected. This is my last resort to get this matter resolved.