Delete duplicate photos

The organizer is built to prevent importing files already in the current catalog. To do so, it mainly checks at import time if the date_taken and the file size in kB are the same. That's a pretty fine criteria to filter photos from your camera but not for scanned photos of various origins.

So, if you try to import the same photos several time you may create duplicates on your folders but not on your catalog. For instance you import from external tools or into a different catalog but in the same folder. Then there is nothing you can do from your catalog since the duplicates are unknown to the catalog.

On the other hand, you may have similarity duplicates from various scans or imports with various sizes or dates: the visual  similarity search will be useful to find them in a catalog.

Using external tools  may be the solution, but understanding why you create duplicates is even more important.

Why did you have duplicate/triplicate copies on your external drive?  Frankly, the Organizer does not have adequate tools to find and delete true duplicates.  (In theory, it should not import them in the first place.)  I suggest that you use a specialized utility to find the duplicates.  If you delete files outside of the Organizer, make sure that you perform a Find>All missing files to remove the deleted files from the catalog.