Duplicate Playlists, multiples of them

Hello! I found the reason why this happens, after a long time...

I will give you an example.

On my iphone, I have a playlist named "Rap". In this playlist, I put a song by Keny Arkana - Marseille from my PC. After some time, I got the album "L'Esquisse 2" from Apple Music. On this album, there was also the song "Marseille", so I had the song twice. For that reason, the next time I synced the iphone, itunes created a playlist named rap1.

You can find the problematic songs verry easy. After you sync the iphone, you enable back the option for Apple Music from settings (you have to disable it for the syncing to work). You go to your Music app and after the syncing with your icloud music is done, you check the problematic playlist (in my case, "rap"). There will be 2-3-4-10-20 songs that will have the cloud icon, which means that even if you got them from the PC, you still got them from icloud too, because you have them on an album downloaded from Apple Music. You delete those songs from the iphone, you delete them from the PC folder (so itunes won't sync them again next time) and you also delete them from your itunes playlist and then you connect your iphone to itunes and sync. You only let the album song on your iphone. Or delete the song from the album from Apple. That's it. I hope I explained it well enough ...

Once possible mechanism is if the same list is updated in iTunes and the device between syncs. Once iTunes has started doing this then it might be best to remove all copies of the affected lists from the device, deduplicate, then sync back what you want. See also this post on erratic syncing.

tt2

Expected no, sometimes observed, yes. In principle you're supposed to be able to edit playlists both in iTunes and on the device as you choose and those edits should transfer in either direction when you sync without creating duplicate lists. In practice this seems to go wrong sometimes for reasons that aren't exactly clear. As I said a possible trigger might be when the two copies of a playlist both get updated in between sync operations, so it isn't clear which version should now be the definitive one. Let us imagine that each playlist has a unique ID, and a time stamp for when it was last updated. There is now a conflict, which one wins? The one in the library? The one most recently updated? Ask the user? Or save both versions and let the user choose later? My guess is that the last choice was made. I also think it might be possible that sometime iTunes might think there is a conflict when there isn't.

You mentioned that the second copy of a playlist might be missing some of the tracks. Is there an optimization feature enabled on the phone that lets it discard media when it is running low on memory? This could potentially alter a playlist held on the device without your active intention.

tt2

Are the all of his tracks ticked in your library? Are they marked as part of a compilation? Are the tracks downloaded to the computer? Do they play when selected in iTunes?

tt2

One of my musical artists - Paul Simon - does not sync when I update my iPhone X to my computer. I am current with my software updates.

I can confirm this still happens with Catalina and the Finder sync. In fact, I also get duplicate folders as well!

I've noticed that it is trivial to reproduce - just sync, delete all the duplicates from your Mac and iPhone, then simply play one song update the play count. For me, that playlist will not get the playlist 1, playlist 2, etc. etc. forever and there is no going back.

In complete desperation, I did the whole "everything in the cloud" using Apple Music, and guess what - zero issues.

Since that's not practical for me, I can only assume Apple does not really support or possibly even test not using the Cloud or USB to sync vs WiFi.