[dev-tacular]

Showing IDs would work, but also training employees not to assume a warning is a glitch would have prevented him from boarding. If they took the time verify that the original person had already scanned their ticket, they would have noticed something was wrong.

EDIT: this is assuming that the warning is not routinely ignored (i.e. there's actually a bug in the system and it would slow down boarding time since it happens so frequently.) If that were the case... they should fix their system lol

[rdtsc]

It might be a generic "beep" error that a dirty sensor or paper would normally indicate. People get used to false positives quickly and start ignoring them.

Maybe it was a different alarm with a distinctive indicate that "I read this fine, but it's a duplicate" and they would stop and start checking the ID.