You can use Duplicate Titles and make sure you leave 1 copy and only remove from Library. You can also sort on Added and remove newly added files.

You likely had Folder Monitoring enabled, this should be disabled when switching drives or computer.

Yes

I may have figured it out....
I used "Duplicate titles" under "Files To Edit".
It listed a ton of files... I then did a select all, and delete.
It asked if I wanted to delete from library and disk, or just library.
I deleted from library only....

so far, things look good.

Does this sound like the solution?

So after moving my video library to another disk and patching the broken path names with "UpdateLocationOfFiles-1.4.3" I ended up with many duplicate videos.
There is actually only one MKV file in one directory, but MM shows two entries.

I suspect the problem came because when the paths were wrong after the move to a new disk, MM rescanned the new disk whereby creating the duplicates.

Is there an easy way to prune all duplicate videos displayed in MM just leaving one?
(There are hundreds such duplicates.)