Authorize.net, the payment gateway, has a function to prevent a duplicate transaction from taking place for the customer's protection. Currently, the duplicate time frame is set for 2 minutes. So, if a customer attempts to purchase tickets, their card is declined, and they attempt to purchase tickets again on the same screen with the same credit card and billing address (all within a 2 minute time period from the initial attempt), it's possible that they will see a message on the screen stating that it is a duplicate transaction.
For more information about this function, see the link below: