It is not usual for a single ticket to be charged twice to your account. Sometimes, a balance check is undertaken which is followed by the actual charge, so that they look like two separate charges.
If you have made several attempts to purchase tickets, one of them might have cause the price of the ticket to be retained, even if the transaction was not finally made. In these cases, you will not see the ticket in the "Tickets" section of your profile and the withheld charge will be released in a matter of a few days.
If after a few days you have not recovered it, get in touch with us.