It probably happens now and then that a ticket is accidentally scanned twice and so this message wouldn’t seem that out of place, and so they probably proceeded as normal without thinking that someone else really had already boarded with the same boarding pass.
Edit: I have no idea what the actual messaging looks like. It could indeed have a timestamp. I’m just hypothesizing.
For example, I suspect a decent number are couples/families that print multiple copies of boarding passes and two accidentally try to use the same one.
I've been prevented from boarding because the airline sent all the party tickets as a single PDF and my confused family members all scanned the first barcode they saw.
We should have baseline expectations for UX thinking which are higher than this standard.
Should be easy enough, UI shows the picture of the guy who scanned the first ticket and system tells operator you go and check his ticket. If you can't find him on the plane he's hiding in the bathroom, call security to get him out.
I've lived in a number of countries in the past 10+ years and encountered more than my fair share of edge cases when flying. On the average, the gate agent has to override something 1-2 times per year when I fly internationally.
Given there are thousands of security cameras in an airport, I don't think anyone would mind if one was used to record who got on the plane.
"He's a witch! Burn him!"
Humans make mistakes. You don’t need to prosecute every single individual that makes a mistake you know