So first of all, I thought you were talking about audio files. But you're actually talking about JPEGs.
But there's something you're not being clear about. You said:
"Regularly we transfer new files to each computer.
That way, a file is only transferred once.
But as said before, tagging is a long run effort and I can tag files from 2014 in 2019.
And I don't want to transfer each time every file."So here's what you're still not telling me... When you tag 10 images from 2014 in 2019, and you want to get that updated info to your father, which of these do you actually do?
A. Copy 10 image files to your father, along with one .MPL file which is the smartlist.
B. Copy 1 .MPL smartlist file to your father, and no images files.
Pick one. Tell me the answer. When I said be specific about what files you are copying, that's what I meant.
If what you do is A, I think I know exactly what is going on.
Now, MC actually does "remember" files you've deleted. Create a smartlist, and when you configure it, hit the import/export button, delete all the text you find inside, and paste this in instead:
~d=r
Click OK and then view the smartlist. That's every file that MC remembers you as deleting. Delete them from that smartlist, and your will erase MC's memory of that file.
One more thing. On the computer that's doing the importing, Go to File->Library->Import->Configure auto import and you will see these two options. Ensure they are checked or not checked exactly as I show you here, and click OK.