Over charrging of MTN accounts

I currently have two mobile contracts with MTN with the same issue on both accounts. My account is paid via debit order on a monthly basis. MTN has charged me double the amount for both accounts in July 2016. MTN claims that my accounts were not paid in May 2016. I have reviewed my bank account details, which clearly show that the full amount for both my accounts have been taken off monthly, without exception. I have been trying to resolve this issue with MTN since June 2016. I have lodged several claims via e-mail and have been unsuccessful via the MTN call centre (no answer). In desperation I have visited the two local stores for assistance. The first store was unable to assist me. The second store has tried to assist me and has sent through all my banking statements showing full payment on several occasions. It seems that MTN has no intention of resolving this issue as I have not had a single response or acknowledgement to any of my requests even though I have several reference numbers. . My first contract has ended and I am in the process of changing my service provider and will switch my second contract as soon as it terminates in the near future.