Very angry client- money deducted twice

I asked MTN to change my debit order date, the tele-assistant did that and then came January and MTN deducted my monthly premium twice on the original debit order date and the and new debit order date, i phoned in and got a very unfriendly person helping me telling me he will escalate the matter, i told him i will be going to the bank to reverse the extra debit order immediately. This month MTN deducts double the amount i normally pay. Why should i have the amount of money deducted from my account without my permission, when i have already paid for Januarys payment. Very ****ED OFF and I want my money put back in my account immediately. Not a good way to keep long existing customers.