Multiple Named Logins on Production

I am getting instances where a named user is sharing authentication details with a colleague on a production system and they are somehow able to both use the system as the same named user. As far as I am aware this should be impossible as the platform should terminate the older session. Is there any case where this would not happen?
1 answers

This is a project setting: go to Project -> Settings -> Runtime (tab) -> Multiple sessions per user (option)