In Windows 10 Settings - Display you can make one Display as the main display as shown from my Windows 10 Settings:
This should only show the display you have chosen as the main display.
Mine shows 3 displays but actually it is two. One to my Computer Monitor, the other is my HDTV, and the third is my A/V Receiver for Surround Sound Audio.
If I turn on my HDTV and select PC then I get a duplicate of my Computer Desktop. The other choice is TV and that is all I see. My Computer monitor is the main Display showing.
You can always disconnect the other monitor's cable or turn of the monitor.