BUILDERS refuses to refund duplicate payment

On 13 December I bought a sander online via Builders. My credit card was debited twice. Since then I have been battling to get my money back. I first had to wait for my official bank statement to be issued and then waited another 15 odd days. I was then told that the duplicate had been reversed back to the Builders account but no one can tell me where. Then I am told if I want the money back in my bank account I need to go to a branch and get a stamped bank confirmation letter. I do not have time to go to the bank so they will just not give me my refund. Why must I be the one that is inconvenienced because of their poor service ethic. And FYi none ever gets back to you!!!