Debit account for payed amount

My name is Mandy. I had a contract with cell c, called to cancel and asked for a pay up amount. I paid but somehow the agent dealing with my case did an advance payment. In short i was told im still owing, and my case ended up with nudebt. I inquired about the amount, paid the full amount and even sent a pay out letter. i was told my account is closed and sent to cell c. however few months back i was told that i am owing, how the hell do i owe when i paid the full amount. i received an email of apology that it was a mistake. Today i view my statement, and BANG nudebt debited from my account again, how the F*** do they do that, they have putted my account to a negative value. That will affect my credit score... the F***en debited twice....i paid the total amount, so why are they debiting again. i am so angry and mad.