Follow these steps:
- Close Final Cut Pro X
- Open Font Book;
- Go to the edit menu and select "Look for Enabled Duplicates" or press cmd L;
- Click Resolve Manually
- Check the box Resolve duplicates by moving duplicate font files to the trash;
- Click Resolve All Duplicates and click ok;
- Close Font Book and Open Final Cut Pro X again.
This process should solve this issue.
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