Ubuntu 16.04 & Nvidia - Adding external monitor stretches single display across both monitors

Press Ctrl+Alt+T to go to a terminal and type:

nvidia-settings

then you'll see something like this screen:

From the left pane,

• choose X Server Display Configuration
• click the secondary screen in the dialog window
• click Configure.
• Enable TwinView.

If it doesn't work immediately, reboot the computer.

I had the same problem and I solved using compizconfig. In General Options → Display Settings disable Detect Outputs and set the outputs format.

I've got the same problem with a Dell inspiron 7567 with a 1050 gtx, drivers nvidia-375.

The only solution I found out reading your answers is that if you unplug your HDMI, logout, login, plug in the HDMI and switch to extended screen, it will display correctly instead of streching one resolution on dual screen.

It's temporary but at least we can use it.