I recently noticed an odd issue, not exactly sure when it started, but I think it was with updating to 8.4, though it may have been updating iTunes from 11.x to 12.x, as I did both at roughly the same time.
I have a set of playlists that I sync to my iPhone 6 via iTunes tethering. One of them duplicates and triplicates itself. For organizational purposes, this playlist is contained in a folder with other related playlists, and is manually managed. Also of note, when I use this playlist, I do so by using the "Add To Up Next" command so that when it's over, my regular playlist picks up where it left off.
When it duplicates, it is duplicating as a top-level playlist, and the duplicate's content is different. The ordering within is different and it usually is missing a few tracks. Last night, I noticed that I had two dupes, each with a different number of tracks each in a different order, neither of which matched the original playlist.