i would go back to the shop tomorrow with your receipt and , if you can , a print of your online banking statement showing the 2 charges
Go back to the store, and show them your receipt, You only bought 1 pair of shoes, but got charged for 2, Show them your bank statement, THEY HAVE to refund your money, and ask for something FREE!! LOL That s what I would do, they practically stole $20.00 from you!! ;)