Payment arrangement not Honored by Nedbank

I had an arrangement for Nedbank to debit R1554.09 + R777.05 which is my Dec 2015 installment and Nov 2015 arrears to be debited 22 Dec 2015. Jan 2016 they supposed to debit R1554.09 + R777.05 for Jan 2016 installment + Nov 2015 arrears. On Friday 18 Dec 2015 Nedbank debited R1554.09 of which I called them and the agent assured me that only R777.05 would be debited on 22 Dec 2015 however Nedbank debited R1554.09 again + R777.05 which means they did not honour the arrangement made. I have other debits going off today of which it clearly won't get paid and I will be charged interest. Who is going to cover that interest? Definitely not me as I kept my side of the arrangement however Nedbank felt to double debit me. I want R1554.09 to be refunded today still for my other debit to go off and if not then whatever interest charges I will be charged I will hold Nedbank accountable and it will only be fair for them to pay that interest.