Everything was fine till I installed KDE desktop environment too, and that is when this started for me. (I installed KDE, so that i can use dark theme in my KDE apps like Kdenlive and Krita, as they were not available after installing 20.04) Gnome terminal, Nautilus, Ctrl+I, Properties have this problem. Maybe because they don't have such regular title bar by default. But now it is added/enabled because of something, KDE or GNOME extensions did. Did anyone find a solution? Check the image:
You can disable the secondary header as stated here by using:
gsettings set org.gnome.Terminal.Legacy.Settings headerbar false
Log out of your session and log back in for the setting to take effect.
I'm on ubuntu 20.04 LTS and it's works for me
PS: I noticed this after the installing of gnome-flashback
NOTE: I myself don't suggest Installing many Desktop environments, Unless you are ready to resolve all sorts of issues raised by conflicting config files. It is better to download and install Ubuntu with pure Gnome, as we can customize it as we want.
I solved my own situation with a little trial and error.
Installing gnome session and logging into it solved my problem. This gave me all gnome customizations I needed too.
sudo apt install gnome-session
Now I can see 4 environments to choose while logging in. Click on the white circle which appears on bottom right corner of the login screen if you want to check how many you have. By default this is hidden, but it appears when we click on username to enter password.