More than one user object is being created for a user. The exact steps to reproduce are not known at this time but the bug is confirmed. The best guess to reproduce.

1 - Create a new AD user.
2 - Log in the first time via HTTP (inside the SI firewall)
3 - Add a concept
4 - Possibly do not log out
5 - Log in as the same user via HTTPS (outside the SI firewall)
6 - If the bug has happened you will not see the concept

Admins can see duplicate user objects if it happened
It is not known if the user logged in with case variations in the user name