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I had 10.10 with 4 users and upgraded to 12.04.1 from CD. (in the installation options it detected I had 10.10 and windows installed and I chose the option to upgrade keeping users and their homes and all possible apps)
Now the main user works fine but there where none of the other users, only their home directories, so I decided to create new users with the same names and seems to worked fine, there was no extra home directory created so I assume it linked the newly created user with the home directory of the same name, but I can't log in. It accepts the password goes black and takes me back to the login screen (lightDM) If I create a new user with a different name it works fine but then it creates it's own home directory.