Paid twice for product;

I am still trying to get a refund for R11900. from 13 January , I bought an inverter online and was informed the transaction had failed so I paid with another card and the transaction was processed: when I looked at my bank statement both transactions had been processed. I have been in touch with BW and sent them all the documents they required, in the past week I have sent 2 emails and have had no reply. I want my money back from them.