MTN issues with contract cancellation

I had a very frustrating experience with MTN I ordered one phone on contract but was incorrectly given two, returned the extra device in-store and followed the full cancellation process as instructed, and was told it would be resolved within a few days, yet two weeks later nothing has been fixed; although I haven’t been billed yet, I’ve received an invoice showing they intend to charge me for two phones, which is unacceptable given I returned one and comp**** with all procedures, and the lack of urgency, poor communication, and failure to correct their own mistake is extremely disappointing.