Two lock screens when unlocking my computer

What is extremely likely happening when you close your lid is the screen-saver kicking in. Therefore:

sudo apt-get remove gnome-screensaver

will solve your problem permanently after a reboot.

The gnome-screensaver is just an additional package that you remove (I'm running like this as well) and has no impact on your overall system (except that the screen-saver doesn't kick in, locking your computer after a period of inactivity).

If you use this computer for personal purposes, that's the only thing you've got to do!

If you use this computer for business purposes and still want a screen-saver that locks your screen automatically, you can still install xscreensaver from the software center (add the optional components for a ton of extra screen-savers).

I was able to remedy this by disabling the lock screen in the screensaver settings on my Ubuntu installation. I have MATE and gnome both installed; I think what happens is you get one MATE lock screen and one gnome lock screen; disabling the one within MATE leaves me with just one (admittedly nicer-looking) lock screen from gnome.

Deactivate the screensaver via commandline:

/usr/bin/gnome-screensaver-command -d