The 1st user logged into his account on his computer, then the 2nd user tried to log in on their computer, it for some reason logs into the 1st users account. And it is then mirroring the 1st users session.
When the 2nd user logs in, it doesn't ask for his log in details eg name and password section. It opens straight to his account.
It doesn't kick off the 1st user off.
It doesn't come up with 'session in use'