Cell C double invoice

I have had various contracts with Cell for well over 10 years. They have a debit order and have deducted from my account every month- I have proof. Nudebt have harassed me for months re R461 I apparently owe Cell C, they could not tell me ANY details how I owe this, as I have paid every month. I eventually contacted Cell C and they told me it is from October 2017, they sent me an invoice for Oct 2017, and I sent them the invoice I received in Oct 2017 PLUS PROOF OF PAYMENT. They cannot explain to me where this other invoice is from, as I have paid my invoice for Oct 2017. Now Nudebt is starting their nonsense again.