Contracts are a joke- they deduct whatever amount they want from your account!

I took out a contract for an iPad mini at the Cresta iStore in January 2015. The contract was through MTN over 24 months at a cost of R119 per month- supposedly by debit order. Each month R119 has been deducted from my account and then suddenly at the end of February 2016 I noticed a debit order for R999 for MTN. When I followed up I discovered that someone at MTN 'erroneaoulsy' logged two separate amounts for deduction from my account which totaled R999. I am shocked!! How is it possible that I have signed a contract of for a debit order of R119 to be deducted from my account each month- a SIGNED CONTRACT- and 'someone' at MTN can simply deduct whatever sum of money they want from my account?! Repeated follow up phone calls with reference numbers to rectify the matter have thus far been fruitless! This is theft MTN!