Here is an addon that will select any duplicate faces.
Select Duplicate Faces 2-8+.py (3.4 KB)
Select Duplicate Faces 2-79.py (3.4 KB)
There is a version for Blender 2.79 and a version for anything beyond Blender 2.8. I made it because of this thread here, so this is it’s primary use.
It uses the origin of the face to determine whether it’s the same as another so it’s not 100% accurate but 95% hopefully. Although, for the use case mentioned above it’s worked every time I tried it.
The function can be accessed via [ View3D > Select > Select Duplicate Faces ] or via the keybinding [ SHIFT+ALT+D ].
Only works in Edit Mode. And it is recommended that you are in Face Select mode.