Double Debits On Contract

I took up a contract with Mtn at their northgate shop. For the past three month's they have been debiting double the agreed amount. There customer care number never gets answered so I went in store last month and was told the double debit was a system error and they would get it sorted. It's been 3 month's now and the double debits are still going off my account. I have lost faith in Mtn as my service provider and regret the day I walked into that store. Mtn customer service is terrible, the staff just don't seem to care. They blame their system for every single thing. .