Charged double for tracker since upgrade to skytrack

I called to query my billing as it has always been high and I recently changed cars and o had to pay a settlement amount for the car I sold. To my suprise am still billed. I was told I had 2 devices in one car which I was billed for since I upgraded from tracker connect to skytrack. Which I think was in 2014 or 2015. Now I am told they can't refund my money for the full duration only for 12 months. I want my money back, it's their error. I want answers as to why I was billed for 2 tracking devices yet they can't pay back the money they took.