There is an annoying bug on Windows Phone 8 that causes the songs on your phone to be duplicated multiple times in the playlists. I managed to fix this by only putting the songs on the phone memory and leaving the SD card for photos and video. That way my Nokia 520 does not have any duplicate songs anymore. This is an easy but safe fix; until Microsoft patch this bug in a Windows Phone update. But this does work very well. There are other fixes suggested; like syncing all of your music to the SD card with Windows Media Player. This does not work though, the music copies across but this leaves you with the same problem. You need to leave the music on the SD card instead. There is a fixed amount of storage space, but this is better than putting up with songs duplicated multiple times. Your phone makes a separate list of songs on the actual phone memory. This is/was a software glitch on the part of Windows Phone. Only an update could fix this issue in any real sense. So if you put all the songs on the phone memory to begin with, then you will not have any problems. Until the bug is fixed, this is the best workaround. Leaving the SD card in the phone is fine for storing photos, documents and videos, but make sure all music is on the internal storage space.
I promise that this fix does work.