My roommate is switching from PSE 12 on one computer over to PSE 2021 on a new computer. He is writing a book about cars und decided to use albums for organizing instead of tags. So he will have the exact same photo, same name and file size, in one album for the car's year, then a second album for the manufacturer, and sometimes a third album for the year he took the photo. In PSE 12, he has about 49,000 photos, but 28,000 of them are duplicates.
Of course, when we went over to PSE 2021, it automatically deleted all duplicates. He is now unable to find some photos because that entire album (of all duplicates) got deleted. The photo is still there somewhere, but under a different album.
It seems the best solution would be to go into PSE 12 and tag all the photos in each album (all the photos in album "1978" should be tagged "1978" and so on), then export to PSE 2021 again. But tagging manually like that will take ages, plus is prone to typos.
So my question is, is there some way in PSE 12 to automatically take all photos from album "x", tag them all "x", and then do that same thing for all albums?
Do you sitll have PSE12 with all its photo files and its catalog?
If so, use the only correct way to move your catalog and photos to another computer:
Create a full backup on an external drive and restore it on your new computer from PSE2021.
This will restore all your files including those you think are duplicates and all your albums. The restore process will also convert your catalog to the new PSE2021 format.
It seems that you have imported your files in a new empty catalog after copying them on the second computer. That does not restore the albums and if you have real duplicates (I am not sure), they will be filtered out.
You have to know that albums are just 'playlists', they don't contain any photo file, only links. You can include a given file into several different albums for different purposes without having any file duplicate in your computer.