Neat folder and file names, correct tags and covers, and the albums you have twice: see every change first, pick the albums, and undo it for 30 days.
In Broadcast: Server, Kinds of media, Music, Tidy up; or Library, Music, Tidy up.
In Classic or Neu: Library, the Music tab, Tidy up; or Settings, Library, Folders.
On the Music page, press Tidy up (or Server, Kinds of media, Music in Broadcast; Tidy up my music under Settings, Library, Folders in Classic or Neu).
Press Check my music. Checking changes nothing, so it’s safe to run as often as you like.
Albums to tidy, already tidy and left alone, each with its new folder and file names, tags and cover. Albums you have more than once are listed apart: Usherr never removes a copy by itself.
Under Ready to tidy, tick the albums (or Select all) and press Tidy. Start with a few. The originals are kept for 30 days.
Optional: get a free application key from AcoustID, paste it in Application key on the same page and Save. Albums are then also recognised by their sound.
Earlier tidying lists each run with Undo, which puts everything back. The kept originals show the space they use, with Empty now.