Hey Michael and thanks for checking in on this!
That issue has indeed been resolved — the latest versions of Event Tickets and Event Tickets Plus will ensure that scanning the barcode of an attendee twice will only result in a single check in. 🙂
There is no error that displays if a barcode has been scanned twice, but you can filter the report of attendees by those who have specifically checked in.
Does this help answer your question? Please let me know.
Cheers!
Geoff