This is being sorted by artist and what happens is that each of those duplicates are for singles by the original artist and instead of placing them under the main (the one with the cover art) I have multiples.
Have you verified there’s no conflicting embedded metadata?
Though it seems there shouldn’t be much of a potential for typos, I’ve seen tons of files with embedded metadata that got twisted by leading/trailing spaces or using similarly looking Unicode characters – resulting in Plex considering this to be different artists.
If you have bad embedded metadata or no means to control it, you might want to use the online matching via Plex/Musicbrainz instead.
You can multi-select all the items belonging together and then merge all of them in one go (select the 1st item, hold Shift, select the last item).
Keep in mind… that’s not fixing the root cause of your problem. If you ever have to start over with your server, you’re likely to have to manually fix this again.