********** sim card

I took a contract last year with MTN for a device only deal. I got my phone in November and started paying for it in December. As from March this year, MTN started to double charge me. I called them and was told by customer service that a ********** sim card was added to my account without my knowledge and without my approval. I did not sign anything and yet MTN started taking money out of my account *******ly. I was told that I should go to the police station to do an affidavit and certify a copy of my ID and to send both documents to MTN. I sent both documents to ********** and she acknowledged receipt and gave me ref number REQ: ********** and told me to do a follow up in 7 days. A week later when I emailed her, she just ignored the mail. I called MTN customer care and was told that the ***** department did not receive my ID. I sent the document again but still no response from the *********** lady. I sent a message on Facebook and was told to send my documents again on the customer care email address. From this I got a new reference REQ: ********** . For the past three months I have been calling, sending numerous emails and messages but no one could give a feedback. From the last email I got I understood that the delivery company has confirmed that they returned the device. Now the staff from customer care says that channel queries need to resolve my case. ********** has sent a screen shot showing the name of the agent who apparently added the additional sim to my account. I realised that it was the same person, ********** to whom I sent all my details in November 2016 for my device only contract. Now nobody at MTN knows who I should contact, customer care says I should contact channel queries. Channel queries forwarded my email to Nokholeji and she again fowarded the email to channel queries amd *****. SO NOBODY KNOWS WHAT TO DO AND MTN IS STILL CHARGING ME *******LY!!! Please somebody tell me what next step should I take?