Duplicate transactions for the same order

Please help! I placed an order through the Boardmans online site on 17 June and received delivery of my goods promptly. All to my satisfaction... However when I made some other purchases afterwards at a shop, I saw that my available credit was depleted with much more than the amount on the order. When I received my monthly statement, it showed that the online purchase was deducted twice from my account. I have been trying to get this fixed over the past weeks by going into the shop as well as by phoning Customer Services, but nobody seems willing or able to help me. How do I get this problem fixed?????