Unfair and unformal

I had a loan from then. I settled my loan with them. In July 2017 I received a bank msg that stipulates that African bank refunded me With 621.40. Why I do not know and 2 days after that amount was payed in my account a lady phone me and told mevto please come and pay my account because it can make my credit score bad which I rep**** they must please check there info and get there facts straight now they harassing me some of the collecters is very rude they do not even want to listen to me so I spoke to a supervisor she ask for bank statements to prove my point I forward it to her and I ask for a costumer service advisor to tell them about the rude service I received they listened to me but never solved the problem and then some one phoned me again and told me it was there mistake can I please just pay the money back as soon as possible which I rep**** if I do have the money I will she told me it is fine I can pay it with no interest. But now they are harassing me everyday and sended me sms'e to tell me that I will be handed over in 20 days. What the hell is going on in that office