And SSO is a single-sign on. This means that it doesn't check for multiple users at once. Yes this might be a security flaw, but I can't remember the other fix other than trying to repair the table or some other users suggests to set all last_ip to the register_ip
With an UPDATE - I don't have this in my head atm.
Note: This also fixes the issues when users sometimes can't register. You can also repair other tables such as chatlogs if the users gets disconnected each time they write a sentence.
This might not be the fix, and I would go for turning the secure SSO off inside server_settings for a fast fix. But if you wan't a better solution it would be using UPDATE TABLE users [...] query. To set the last_ip to the same as registered ip.