KFC refusing to pay me back

I made a purchase at KFC in Canal Walk on 18 June. I used tne self service points and tapped my card but the transaction wouldn't go though. One of the assistants said I should print a slip then pay at the till which I did. I noticed a week later while going through my statements that tne transaction went through twice. I went oersonally to Canal Walk to enquire about this but the manager refused to refund me my money even though I clearly showed her tbe transaction on bank app and they could see the transaction on their side of the exact same amount on the same day. I want tjus investigated and I want my money back. I don't appreciate being treated like I'm lying when I had clearly showed that they charged me twice.