The ticket barcode has been scanned previously, either by your device or another device scanning the same event.
Ticket History can provide some clarity on the issue. To access Ticket History, tap the error text on the screen or track down the barcode from your Guest List. Tapping the “View History” will provide a list of when the ticket was last used.
If the time, date is very similar to the current time and date, you or someone else at the same entry point may have already scanned the ticket. If you have signed in to the various phones with different users, you will be able to see who scanned in the last ticket.
If the above possibility does not make sense in the instance, there is a chance the person is trying to scan in a duplicate ticket. If someone has presented a duplicate ticket, the ticket is not valid for the event. You may want to find out where the ticket holder obtained the ticket.