Have been facing the issue ever since I got the phone.
Every time I connect the phone to PC, and copy a file in Music folder, I've got duplicate files in my Music app on phone
Some posts suggest that it is due to a setting "Xbox Music cloud collection" being turned on(Settings--> Applications-->Music+videos).
However, turning off his feature does not help, as duplicate file still arrive. Also, it does not explain why such a setting would create duplicates of Photo/Pics
An odd observation:. I've created 4 sub-folders in Music. Whenever I copy a new song in 1 of the folders, the songs of only that folder gets duplicated, and not of other folders.
Duplicating of files is true for photos too. I had 8 duplicates in the phone.. Cleaned them by copying all the stuff onto PC and deleting from phone
Haven't touched the photos folder using PC for a month now, and hence have got no duplicate file in there.