Happening to me as well. I have 50 GB of Synced Content (all music) and 125 GB in Music, which includes all the Synced Content.
One interesting thing I noticed: while this shows up as duplicates in the iPhone Storage page, you get the actual free space in General > About > Available.
For me, there is a 50 GB difference between the 2, so it’s counting the Synced Content as double under Storage but you get the actual free space available in the other page. Surely this will be sorted out in an update.