Being charged for a line I don't know of and no feedback from store consultant and management

So I upgraded my contract on 07 dec 2017 when Mtn was promoting two phones for R299 per month only to receive my statement in January stating that I have an extra line and you are charging me R649 per month for and extra sim plus the original R299 ? I went back to the branch to query my account and consultant advised that they used the incorrect codes and billing will be corrected with the next billing cycle .yesterday my account was debited with the incorrect amount once again and no one from MTN is able to assist me ? when I called the call centre they advised that the ref. no given to me by the store consultant is invalid and that there is no trace of any query on my account .... really this is pathetic I don't have the time nor the energy to go back and forth to a store with *********** staff .