I consistently get two login screens

Solution 1:

Uninstall your gnome-screensaver,

sudo apt-get remove gnome-screensaver

Then, reboot. It works for me, now only unity login screen appears.

Solution 2:

1. Try sudo pstree

   You should look for the Xorg process and see what parent it has. Is that parent process lightdm?

   Then you're using the LightDM display manager, and it's proboably a LightDM issue.

2. Install another display manager, GDM

   sudo apt-get install gdm
   

   During configuration of the package, set GDM as the display manager.

   Does the problem still exist? Now you're sure it's a LightDM issue.

3. sudo dpkg-reconfigure lightdm
   

   Now LigthDM is back.