Two Login Screens with different accounts on both
The first login prompt is for File Vault and shows the administrators that can unlock the machine, the second prompt is to actually login to the device and thus can be any user including guest.

To remove the double login process you will need a single administrative login account.  This account needs to be new or the password reset using the following method:


  Settings > Security &amp; Password > Change Password ...


Alternative is to completely remove File Vault and thus one could login to the non-administrative account.  File Vault 2 is an all or nothing affair.