Duplicate session id error in database table

I'd check if expired sessions are being properly cleaned, especially if you're running Debian.

Thanks, Narf. I checked the session table for distinct dates and it doesn't look like we have a problem with expired sessions.

Good news though--we haven't had the duplicate session id issue for the last 3 days. I wanted to observe the site for a few days before I posted this update.

So we added a call to session_write_close() in a function that generates PDF (via TCPDF) and serves it for download. Before we generate the PDF we set a specific variable using session->userdata so I noticed that practically all of the duplicate session id cases had that particular variable in the data part (as seen in application/logs). Then something about what natanfelles said regarding the lock check failing made me wonder if the locks were failing (?) for some users during the PDF generation, that's why we thought of just closing the session altogether right after we set the session variable and before TCPDF gets to work.

We're still not sure if that's what was actually causing the issue and if closing the session was the right move, so we'd appreciate any insights on this. For now, we're just a bit relieved to not see the issue pop out in the logs for a couple of days.