INCOMPETENCE!!! WORST SERVICE!!!

I had 2 decoders and one was returned months ago. I've been paying for the one I have without any issues, last month I decided to pay a month in advance. So I paid for 2 months, the previous month and for this month. Yesterday I got disconnected and they are telling me that both decoders were active so they allocated the payment I made in advance to the decoder which was returned to them. Now how the hell am I paying for a decoder I returned to you guys months ago? Please sort this out expeditiously. This is annoying. I paid in advance and this is the treatment I get?? Come on!