How to fix Unity Editor multiple "DX11" windows
To fix it just select in the Unity Editor: Window / Layouts / Default
I decided to make this video because, when this problem firstly happened to me, I couldn't find any related answers on YouTube or even Unity official forums. I even thought it was some kind of video drivers issue, as I experienced the same duplicated windows with VLC media player, but probably it was just a strange coincidance.
Hope it'll help :)