Normally tickets that are scanned at the entrance and are classified as valid, cannot be scanned again. When valid tickets are scanned again, the error Already checked in appears on the screen of the scan crew.
In some cases it can be necessary that tickets need to be activated for scanning more than once. For example, when your event has separate areas at the event site. When multiple scanning for one ticket is activated, the ticket can be scanned multiple times. This article gives a step-by-step explanation on how to activate multiple times scanning of a ticket.