Douyara Watashi no Karada wa Kanzen Muteki no You Desu ne - Vol. 9 Ch. 54

[chomikko]

Either one shot incoming or something that shall not be harmed will appear.

 

[flannan]

I wonder how the mirror works... Can we generate infinite human organs this way to do medical operations?

As long as the recipients are fine with living within a fixed distance from the mirror. Which is a rather restrictive disadvantage.
And you'll have to find a person whose prank!double is fine with donating organs.
And there is a chance the organs have mirror biochemistry, fairy biochemistry and many other potential problems.
Overall, there is probably an easier way to generate human organs.

I thought their personality were differents... wouldn't that normally include their fear of heights ? That mirror changing things around but only partially is weird.

It's not that the mirror images have a completely different mind. They are still very much Mary and Magilika, but just with a few "fun" changes.
This is actually common with evil twins, where the mirror image would be the same as you, but evil.