Official comment
If the relocation fails because the song "already exists in the collection," that indicates you have imported the song once, then the file was moved, then imported again, leaving the first import with the source file "missing." Relocating that file won't be allowed because the file is effectively already there, you have a duplicate.
My recommendation is to find one of the missing songs, then sort the collection by song name, and compare it and its twin. Look in the info panel or show the column with the path information to note where the old file was and where the new file is located. See what change was made, then look at another pair and compare those two changes. Figure out which one you want to keep -- if it's the new one, you're good, just delete the song with the missing source. If it's the OLD one, then you need to delete the new entries (you can take note of the import date of the new songs and then sort your collection by import date, then batch delete those entries), and then use this process to relocate the songs.