Unfair hand over on unknow debit order

For the past few weeks cell c has been trying to take money out of my account in a form of a second debit order and I asked my bank to reverse it because I only have one account with them, then later I was told I was handed over to nudebt for an amount which I have no clue where it comes from. According to them I have an overdue amount that needs to be paid. Now my question to them was how do I have an overdue amount if you guys are debiting me every month and I've been with my bank for over 15 yrs, never defaulted on my debit accounts payments and they are still debiting me yet they handing me over. I have never lost an id document. They need to resolve this problem cause they are stressing me out with something that I believe is their fault.