Problems syncing some songs

Hello, everyone.

I have been facing a very frustrating issue upon syncing music from iTunes Library to my iPhone. It has happened before, and I managed to solve it in a very roundabout way which I unfortunately do not remember.

There are two artists in my Library who are not syncing properly. One of them is missing a single track, the other is missing nine tracks. All ten tracks are from purchased albums (the other purchased tracks sync just fine). iTunes syncs 789 songs, skipping these ten tracks. Please compare:

On the 'On my Device' tab, there are 800 songs listed. I have only 799 tracks in my Library, which means that there is one duplicate song. Now, I've read that iTunes stops syncing when it encounters a duplicate; this makes sense for this one track, but not for the other nine tracks, which do not have duplicates. The tracks that do not sync are greyed out and have a dotted circle next to them. If I delete them, they just keep coming back.

These are the greyed out tracks which appear on the 'On my Device' tab:

The track 'Torn Apart, pt. II' appears twice (sometimes, however, the entire album is duplicated and does not sync), once in its original form and once in my stylised version (I am very, very picky about my Library). I only change info such as artist name and album name, the file remains the same. As for Slipknot, nine tracks out of their new album do not sync, despite not being duplicated.

I have tried everything I could possibly think of:

• deleting all songs (both directly from iPhone and from iTunes, unchecking 'Sync all music') and resyncing
• checking 'manually manage music and videos' and deleting the greyed out tracks
• turning the two albums into compilations
• changing the artists' names
• deleting both artists directly from iPhone and resyncing
• restoring a backup

This is driving me absolutely insane, I nearly passed out from anger. I do not understand why only these ten tracks in particular are causing trouble.

I am using an iPhone 5 running iOS 8.4 and iTunes 12.12, though I'm not sure these specs count much as this issue happened before with a slightly older version of iTunes and iOS 8.4 beta.

Hope you all can help me out. Thank you.