Fonts aren't showing up in Font Book
There aren't ANY fonts showing up in my font book, not even default fonts. And when I try to install custom fonts, it doesn't let me as my screen is stuck showing a pop up that says "Multiple Copies of this font are installed" but all of the options are grayed out. I don't know what to do??? If anyone can help it'll be great! (Also this is just font book. In other places like Word and Photoshop, fonts are showing up fine.)
Here is a screenshot so you can see what I mean:
Notice the grayed out notification box, and the fact that it says "All Fonts (0 Fonts)" on the top. Not even default fonts like Times are showing up.
MacBook Air, macOS High Sierra (10.13.6)