5th Month in a row being double charged at MTN

Today was the 5th time, I had to take off work, sit for an hour in a MTN shop to rectify my account with them. For 5 months in a row they have Double charged me for data. Every month I get credit for it and every month they just do it again. How is this possible that a simple problem like this can't be rectified? Thank Goodness I check my billing every month.