Not sure if theres any special way of doing this, but I take it your checking for duplicate users every time someone logs into the system. If I am correct in assuming this you could simply add a boolean field called 'logged_in' to your database where you check your passwords. eg.
user already logged in = 1
user not already logged in = 0
Check this field every time a user tries to log in. If its set to 0 log the user in and set the field = 1. If the user is already logged in (field = 1) redirect the user to another warning page.
Not sure if thats any help just made it up of the top of my head there now.