I'm a new user, strictly beginner. I'm assuming my terrible keyboard skills have seen me trigger a hotkey to undock the image from the Loupe view in the Develop module. The image displays in the docked window, but there is also an undocked copy sitting on the screen i can't get rid of. I'm sure half my problem is not being used to the terminology for different window types so I can't search the right fix.
Screenshot for example - it is the smaller copy in the top right I want to close.
You have enabled the secondary window option. To disable it, click on button labellled '2' just above the filmstrip on left side. On your screenshot it located slightly off centre above first image on the filmstrip.