Over debit my account

If I could give zero stars I would. I have a personal loan through nedbank and due to a change in circumstances have an agreement in place to pay off the loan. Ndcrs failed to debit my account in September and claims there was insufficient funds, so this month they did the usual debit aswell as an additional amount equal to 4x the agreed monthly debit. The call centre agent claims if a debit is not completed, they will debit any amount they see fit when money is available. I find this behaviour unacceptable as an error was made on their side not mine, so I should not be punished for that.