Refund my money

My debt of R1600 was handed over to MBD without my knowledge and I payed and payed until I realized that I have paid more than what I owed them. I have proof of the R7999 that I payed. Mbd is still harassing me saying I owe R1400. From R1600 to R9000 dept how is this possible. I want my refund and my paid up letter.