Double monthly charge 3 years and going

I am a customer for the past few years as my wife requires oxygen at home, we have been refilling at the branch in Roodepoort but during the refill we did in 2018 there seems there was some error and we were charged for 2 cylinders instead of 1. I have made numerous calls, visits and emailed customer services but I have and still am paying monthly for 2 cylinders... When will someone actually be able to resolve this as it should be extremely simple as the cylinders are scanned in and out? This is basically theft on your part as you refuse to correct the matter.