Collecting for a paid up loan

I had a loan with FNB which went to areas - It was handed over to DMC and i paid my account with DMC and they even sent me paid up letter. To my surprise DMC only paid 2k on that loan, FNB debited my account to pay the same loan that has already been paid. Please fix your issue with DMC as you are the one who handed over my account. I can't pay the same account multiple times.