Duplicate songs on iPhone 12 Pro Max after syncing

You would think, but no such luck. I do the same thing, backing up to the PC. I just have more info on my phone than the cloud would allow me to back up, so I just do it on the computer. I think I found a way around it though. I unchecked the "sync all music" box and synced my phone. Then I went to Settings-General-iPhone Storage. I hit the Music section and there's an All Music section and a thing for each artist. Next to the "Music" label above the all songs part is an edit button. I hit that and deleted all songs. They still showed up on the phone, but as downloadable songs rather than actually being in the phone. I synced my phone again after I deleted all of the songs and it put the songs in the right places with no duplicates. None that I've found anyway. I have over 10k songs, so I didn't check one by one. It put the songs in their rightful place instead of downloading them or putting in a duplicate where one is there and the other is a download. Give that a try and see if it works. I don't think you have to uncheck the sync music box since you don't have to sync it before you delete everything. Unless you have songs on there that you purchased but aren't on iTunes yet. Otherwise, just go into your settings, delete all your songs then resync your phone to iTunes. Its not ideal, but by that point, I was desperate and figured I'd give it a shot. Lo and behold, it worked. Hopefully it works for you!