There is a difference between display "cloning" and "mirror" mode.
Cloning means same x,y coordinates but different resolutions, so the displays only overlay/intersect.
Mirroring means that the resolution AND refresh rate has to be identical. That's how X11 defines mirror.
So if you have different refresh rates you can manually achieve something like mirror by letting the displays overlay and by selecting the same resolution.
There is a difference between display "cloning" and "mirror" mode.
Cloning means same x,y coordinates but different resolutions, so the displays only overlay/intersect.
Mirroring means that the resolution AND refresh rate has to be identical. That's how X11 defines mirror.
So if you have different refresh rates you can manually achieve something like mirror by letting the displays overlay and by selecting the same resolution.