Outrageous

I have a personal loan with Nedbank and have an arrangement with them with has just been updated and extended for a further 6 months, the debit order is to come off on the 28th of every month, on the 27th March they debit my account for double the amount which is crazy, this happened end of january as well, I dont understand so i called the collections department 5 times only to be told that I would get a refund in 3 - 7 days, now this is my money that they have *******ly debited from my account and they earn interest from it for 7 days and I have debit orders that will bounce, to make matters worse I spoke to a team leader who put the phone down on me, I now sit with no solution and wonder if I will get a response to this letter