I use to have that problem, have you tried syncing your device and backing up??
The only other thing is going through your itunes libary on your computer and deleting all the duplicates on there and then re-syncing your device.... if not go to music under your ipod on the computer and tick a box that says something like sync only selected songs, tick the songs you want and then sync your device..
One of these has to work for you because all of the following has helped me in the past when it happened to me...
hope i helped