Failure to refund duplicate payment

I made a purchase on the 9th of Feb 2016. The till showed the my card declined, however it went through on my side. I then paid cash & I was told the bank will reverse the money within 24hrs. 8 days later nothing happened. I took my statement plus the till receipt to the store to ask for a refund as they recommended. mysteriously the amount doesn't appear on Checkers card payment system. The manager then called my bank which confirmed that the payment went through. still I didn't get my refund coz they say they never had any surplus on their till. what kind of proof should I brings other than my bank statement & the bank confirmation. It is just a small amount to make things worse. I'm not too concerned about the money but the level of incompetency on your side.