Bug#600706: gdm3: I can not get into my account if already open

[diego.fogl...@alice.it]

 this is the problem.
- gdm start
- I log in in one account
- Change user
- I log in in another account (the first account is open)
- I exit to the second account.
- When I try to log in in the first account (was open), gdm come back to the 
log