Why was a ticket buyer charged twice?

Before actually charging the ticket buyer, the payment processor makes a temporary connection to the credit card. This process is similar to pay-at-the-pump gas or hotel pre-authorizations. These pre-authorizations are generally in place for only a few minutes but credit card companies can be slow to take the charge off the actual the buyer’s statement resulting in what appears to be two charges. 

In most cases one of these charges will be reversed within a few days.