They had different names like (1) if they were duplicates. But all my files were named something like "Artist - title.mp3", and the Metadata tag for artist and title were generated based on the filename. I don't know how they would look after being synced over to my phone from Spotify though, but it is probably how Spotify names the files that broke it on my phone.

All the files were in the same folder on my pc too, for reference. So they have unique filenames, but two files could have the exact same artist and song title. I give that's it's an obscure edge case, but really frustrating.