Font Book preferences
Specify where fonts will be installed and how duplicates are handled. To open Font Book preferences, choose Font Book > Preferences.
Default Install Location
Choose where to install fonts:
System fonts you download from Apple aren’t installed in these locations.
Resolve duplicates by moving files to the Trash
Font Book moves duplicate fonts to the Trash when it resolves duplicates. If you don’t select this option, duplicate fonts are disabled, but not removed.