When my desktop recently crashed, the backup created duplicate images in my photoshop catalog on the new computer. How can I remove around 5,000 images en bloc?
Which kind of backup?
A backup from the Organizer?
The backup folder does not contain duplicates. The restore process can create duplicates if you have already copied your files to the new computer from another backup solution.
To solve your problem, you need to explain exactly what you did.
If the duplicates were already present when you restored from the organizer backup, you have duplicates on your drive but you should not have duplicates in the catalog itself. Can you check that?
Assuming this was the case and that you don't want to keep photos on your drive not present in your catalog, the solution would be to make a full backup, delete all photos from your computer and restore from the backup.
For safety, I would recommend restoring on a new master folder in a custom location on your main drive if there is enough free disk space; otherwise on another internal or external drive.
Since it's a natural process to do full backups regularly, there is no risk. You can have a better view of what is duplicated by comparing the before and after photos in your library before deciding to delete the old one from the Explorer.
If you are curious, you can even duplicate (with explorer) the resulting catalog folder for a test. You use the duplicate catalog by double clicking on the 'catalog.pse17db' file in the catalog folder copy. That opens PSE2019 with the duplicate catalog. You now 'import' from the old master folder with the duplicates. The organizer will only import non duplicates and reject true dupes (same size in KB and date_taken). All those imported files can be found from the option to sort by 'Import batch'. You select them and assign a common keywords such as 'new from old'.
You might find that there was some interesting files missing from your old catalog.
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"I thought that elements 2109 would just uses one set of photos from the
files backed up. Unfortunately it has stored both the older back-up versions
of some photos plus newer versions of the same photos from the later backup."
If you did not use the organizer to backup and restore, Elements has stored no file since it only stores links to the files you recovered externally on your drive. If you 'imported' all files from the drive into a new catalog, the import process scanned all your files and ignored true duplicates (files with the same date_taken and size in KB). The file name is not a distinctive criteria. Files with visual similarity are not filtered at that stage, but the organizer has a tool to find them after the import.
That's not very practical.
There are external tools to deal with duplicates, but the difficulty is to select the good version and delete the bad ones. The import process in the organizer only keeps the first found file, and rejects the following duplicates without taking account of the location in the folders tree.
Instead of deleting duplicates immediately, it may be interesting to stack the suggested dupes. That does not recover diskspace, but that makes visual searches much easier. You only select the keeper when you need to use it, one by one. That's not the expected magical trick but that works and makes your library progressively cleaner.