Double billed

On the 24th March 2016 I booked my car in for brakes to be replaced. When I was being billed I used immediate internet payment (IPP) on my internet banking to pay for the service. But it was late and the IPP system was taking too long, so I paid using the normal payment. It turns out both payments went through. I then mentioned this to the service agent at Audi who acknowledged the double payment and she said the accounts people in her company should be able to reverse this. After a number of telephonic and email communications to the agent and the accounts department, they still have not reversed this in 2 weeks. I'm getting really concerned that I will never see my money again.