WHAT IS GOING ON HERE?

I paid my RCS loan in full. However, they keep on telling me that they have not received my payment despite sending the proof of payment to them multiple times. I double-checked that I paid into the correct account with the correct reference. I already received my paid-up letter and the debit order mandate from their side has been cancelled, though they are still wanting monies from me. Please stop harassing me, as I settled this loan more than a year in advance.