Lightroom 2015.9 creates strange duplicate version of folder for HDR/pano images

I have a problem regarding how LR 2015.09 presents and handles my MacOS 10.12 file system. The same folder appears in two places in the Folders panel, with different contents shown. Using "Show in Finder" points to the same single folder, though, with all the images.

When I import photos, they arrive by the method I've used for years in a folder hierarchy of YYYY/YYYY-MM/YYYY-MM-DD. However, when I create HDR or panoramic images, they show up in a separate YYYY-MM-DD folder entry lower down the panel. (The folders are all stored on my Mac's built-in SSD in /Pictures folder.)



If I try to manually drag photos from the original folder to the newer one, or vice-versa, LR stops me with an error message: "File already exists at the destination."



If I click "update folder location" on the original folder, LR "moves" all the photos to the second entry farther down and removes the original folder.

If I click "update folder location" on the LR-generated second entry, nothing happens. If I click "show parent folder" for the second folder entry, it adds a YYYY-MM folder. If I click "show parent folder" on that folder, I get an error message:

Database "/Users/sts/Pictures/Lightroom/LR6 full archive.lrcat": UNIQUE constraint failed: AgLibraryFolder.rootFolder, AgLibraryFolder.pathFromRoot

  Statement: UPDATE AgLibraryFolder SET rootFolder = ?,pathFromRoot = ? WHERE id_local = ?; (error code 19)

I fear that something in the LR catalog was corrupted during a recent OS reinstall, but I can't be sure. It also could have occurred when I moved to a new Mac a few weeks before that.

I tried searching forums for similar problems but I'm not sure anything matches. This one looks similar on the surface but I think it's different: https://forums.adobe.com/message/9354068#9354068