Only affect the default theme, Adwaita.

Open your Adwaita xml (/usr/share/themes/Adwaita/metacity-1/metacity-theme-3.xml) file and change the line that configures the maximized window (name=”max”). Add has_title=”false” to the “frame_geometry” tag. Then look for “title_vertical_pad”, and change “value=8″ to value=”1″. Restart Gnome 3 and maximize a window.

Interestingly, you can still drag down from a menu or button bar to return to a normal sized window and get your duplicate titlebar back.

something like this <frame_geometry name="max” has_title="false” title_scale="medium” parent="normal” rounded_top_left="false” rounded_top_right="false"> </frame_geometry>

How to restart Gnome quickly

Restart the shell by pressing ALT + F2, then r, then Enter