MacOS – Second screen mirror one desktop

https://support.apple.com/kb/PH18757?locale=en_US

This link explains it. Its a new(ish) feature. You can zoom to mission control (f3) or for those with a MacBook, and you have new haptic track pad, you just three finger scroll up and if you move your cursor to the top right corner of your screen you will see a plus sign appear. Click on this to add a new desktop.

You can get rid of it by Going back to mission control again (f3), where you will see all open apps, and you can just close it (them) by hovering your cursor over it and an x will appear on top left corner it, or anything else, out from there. If you're in mission control it's easy to inadvertently create additional desktops if you thro your cursor up out of your way.., I've done it a few times.

It can be super useful when you want one open for work and one for play, but that I guess is not the point. Easy to get and get rid of though. Many good publications on how to make most of the feature if you ever want to.

It has to do with the 'separate space' feature for external displays Apple introduced in OS X 10.10 Yosemite: If the setting "Displays have separate Spaces" in System Preferences -> Mission Control is activated (see https://support.apple.com/kb/PH18809?locale=en_US), every time you hook up a display a new space will be created for that display. After you pull the plug, OS X will not delete that created space, hence accumulating lots of spaces over time.

If you don't activate the option, you're external display will behave like before 10.10: only one menu bar on the main screen etc.