You would hope... But they often run the scanners in offline mode (e.g. at temporary / seasonal events) so there can be lag in the backends being updated.
Heard from a friend who got straight into two events in the same city recently - they presumed the show was at one outdoor venue but the scanners let them straight in at the first (wrong) venue. Went to the correct venue and got in there without any issue too (this suggests one or both venues were offline or using offline scanners).
Hm. So I guess at a small venue that has 3 door people with offline scanners, you have a 2/3 chance of success if you're the second of two people sharing a barcode. Combined with the obvious 3/3 success being the first person, that averages out to 5/6 chance if both of you (oblivious to each other) schedule your arrival similarly.
not really offline but someone who works in industry here once detailed out that each scanner has it's own copy of a SQLite database that is being updated as fast as possible based on inserts of other scanners since any downtime is a big deal at these venues
i.e., theoretically duplicate tickets would be identified but not instantly but still pretty quickly
Heard from a friend who got straight into two events in the same city recently - they presumed the show was at one outdoor venue but the scanners let them straight in at the first (wrong) venue. Went to the correct venue and got in there without any issue too (this suggests one or both venues were offline or using offline scanners).