Harassment for duplicate account

I had a Standard Bank cheque account I believe it has been sold to Nimble group and MBD . Bayport paid the whole balance to MBD and I've got a proof of payment. Nimble group demanded the payment also and I told them several times that I had one account with Standard Bank and it has been paid and I sent them the proof but they keep on harassing me saying it has a different account number. I don't know now how to explain myself to them. I need assistance.