Matrix Telematics debtors department incompetence

I had my matrix MX3 tracker upgraded a few months ago. When they did that upgrade they did not cancel the old device I already had and instead billed me for 2 devices on one car. I've been calling to rectify the issue for months but I get put through to cancelations and end up holding for a long time with no resolution. Now I am being sent an invoice telling me I owe R800 for a device I don't even have. How am I being billed for 2 tracking devices on one car and how am I to pay when the mistake was on Matrix and not me? I lodged a query and was given a reference number but nothing was ever done. Followed up on it and was given yet another reference number but nothing was done after that. I've made query after query about double billing on my account yet no one is able get get back to me to sort it out yet they are able to send me emails about outstanding payments. At this point I don't even know the true reflection of how much I actually have outstanding because I am being double billed. My payments were all up to date until the double billing started. They can send invoice after invoice but don't have the common decency to respond to the queries I have made about the double billing on my account.