I managed to fix this issue on my own. I did not have this issue before upgrading iTunes to the most recent version (12.7.5.9), so I decided to downgrade iTunes. I wanted to share this solution in case others might find it useful.
Warnings: This is an advanced solution. Do not proceed if you are not confident with your abilities. This solution may result in data loss. Back up everything before preceding, just in case you need to undo these changes. Specifically, be sure to back up your /Users/<username>/Music/iTunes folder. Proceed at your own risk.
Here are the steps I followed:
- Close iTunes.
- Navigate to /Users/<username>/Music/iTunes — where <username> is your username.
- Delete the following files or move them to a location outside of the iTunes folder: "iTunes Library Extras.itdb" "iTunes Library Genius.itdb" "iTunes Library.itl"
- Open Terminal.
- Type in: cd ~ to make sure that you're in your home directory. The character after "cd" is a tilde. Then press [Return].
- Type in: cd Applications and press [Return].
- Type in: sudo rm -rf iTunes.app/ (make sure the syntax is correct), then press [Return]. It will prompt you for your password. Enter it and then press [Return]. (This step deletes the iTunes app from your computer.)
- Go to the official page and download iTunes 12.6.2: Download iTunes 12.6.2 for macOS
- Once it has finished downloading, install it.
- Open iTunes 12.6.2, go to File > Add to Library... and select the whole /Users/<username>/Music/iTunes directory. It will add everything back.
- At this point, if you have unofficial audiobooks (e.g. from Librivox.org), they may be incorrectly filed under "Music" in your library.
- Find them in the Music section, select them all at once, then press cmd+i
- A dialogue will come up asking you if you want to edit multiple items at once — confirm that you do.
- Select the Options tab at the top, then under the dropdown menu labeled "media kind," select Audiobook. Click OK to confirm.
- You will now find those audiobooks in the Audiobook section of iTunes.
After completing these steps (no system restart necessary), my audiobooks were not showing duplicates anymore.