I sell software and have recently been contacted by a company that says they have been charged twice for software purchased from my web site, the software sells for $1000 USD. They did purchase one license over a year ago, they have sent me what is basically a credit card statement which shows a second purchase six months later (11 months ago) for another $1000, the statement doesn't make it obvious that the purchase was made via my site but it is possible, I guess they could ask their bank/visa etc to verify this, I've asked for further proof.
I have no way that I can charge their card, I use a payment processor and never see the customers details. To be charged twice they must have purchased again themselves. If they do turn round and prove that the purchase was made at my web site, then they must have purchased again I can't see any other explanation for it. My question is do I have to refund this purchase?