This issue was happening on my iPhone 6 and on my new iPhone 7. MacBook Pro 2009. iTunes is 12.5.5 I have it set to sync entire music library. The issue is everytime I sync my iPhone to my MacBook pro, it resyncs the same 2822 songs (I have close to 4k songs). The songs are still on my iPhone and do not appear to be deleted. The problem with this is when i want to add new songs I have to wait for it to go through the entire sync. I have not changed any settings. The only thing I can thing of is that i toggled between the icloud and this computer back up options on the summary iPhone page. Any help is much appreciated.