desktop shadow or duplicate desktop?

I was showing someone the shortcut for making screenshots and I must have done something wrong. I ended up with a shadow or duplicate desktop complete with the shift key and the command key visible on the desktop. I’ve rebooted but can’t cure the problem? There appears to be 2 screens, one behind the other slightly offset. Neither screen is movable.

on closer inspection, the duplicate is a screenshot, totally opened and unmovable

Could you post a screenshot of this?