Me again. Mine is visible no matter what am inporting from. Are you sure the pane is open with a hard drive, the one named "File Handling"?
You're right! I tried making a new catalog as a test. After hitting "Import" I looked at the right side panel and sure enough, the File Handling was there. I had not expanded it when making my big catalog so I did not see the check box for duplicate files.
Now I have to figure out how best to go back over the catalog and import the duplicates, without having to redo the whole catalog. Maybe "Sync" will let me do it.