If you have received only one confirmation email and can only see one completed order within your account, this means that you may have attempted two payments but one did not complete.

It's relatively common for banks/card issuers to reserve payment when a purchase is being made, so in effect it appears that the funds have been deducted from your account twice, but as the second transaction did not complete, they will reallocate the funds to your account. The length of time this can take varies from bank to bank, but please rest assured that as the second purchase did not complete, no funds would have been taken or sent to the seller.