Double billed

I have been with Mtn for over 20 years and had good service, but lately it's all gone to the dogs. They double billed my account this month as the debit order was deducted three days later than it was supposed to have been.Every time you try and resolve the issue with a call you get a new consultant who you have to explain your problem to over and over. I have tried emailing there customer service and get the same results even if I reply to there mails directly. Eventually I just let my bank reverse the double billed transaction and went and paid the correct amount at an instore branch. Guess what they still say I owe them one months account in arrears and are going to probably double bill me again. How pathetic the system has become.