Well, my first step would be to call the location where this occurred and ask for a manager. I'd then explain the situation and ask them to check their records. If you can remember the precise date and time, then any reasonably competent manager SHOULD be able to confirm that via their credit card transactions, which are usually kept in-house for at least a month. Barring a call or a personal visit armed with either a CC statement, or perhaps the receipt, I see no other solution than to contact YOUR credit card company and let THEM deal with it. After all, they're probably earning a pretty penny off you, might as well make 'em work for it!
Simple, bring the statement that shows you were charged twice or take a screenshot of the transaction and show it to the manager at the location.
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