Paying for an unknown contract 3 years and counting

MTN has a 2nd contract under my name which I initially had no knowledge of! They charged me the full contract amount (R450 monthly) for this other number for 2 years (Feb 2014 to Jan 2016). I queried this and was told they will put a stop to it and refund my money. For the past 18 months (since Feb 2016) or so I have been paying an extra R250 for my contract which I had assumed was for insurance (I had requested insurance on my upgrade). My phone broke and to my astonishment when I took it in for repairs I was told I do not have insurance and the additional money has been for keeping that other UNKNOWN line active!!! When I go to MTN stores I am told they can see that the data/ airtime that MTN allocates to this number has never been used (because I do not have that number!!!). This has been going on for over 3 years now. I have been to several MTN stores to try and get this fixed, I have submitted the required affidavits as well as certified ID copies numerous times at various stores but MTN has not refunded me and the payments are still going off every month. It has become really frustrating! I cannot wait until December when this contract expires and I can leave MTN for good! How do I put a permanent stop to this and get my money back from MTN?!