Vehicle Financing double debited my account

I called Absa vehicle financing to change my debit order date from the 30th to the 15th of each month which their debit time was 2 weeks prior to month end. I never skipped a pay with them but in July they debited my account twice on the 15th and again on the 30th. Obviously the second debit was rejected. In August month end they did the same thing and they received a payment of the 15th, and once again it bounced. Guess who is being charged R150 for rejected debit orders. In September, the same thing happened, double debited on the same day which is the 15th and they got both payments. When i called to change my debit order date, I was told that it was changed and will be effect as from 15 July 2017. Now I am being told it was never changed on their system. Whose fault is that. Now all those months I was changed R150 each month sometimes R300 a day for their mal-administration.