This is a problem that has been reported to me by many users, and that I’ve seen fairly often. In the iOS Music app, if you browse your music by Genre, you may see some genres display more than once:
When this happens, there’s really nothing you can do short of restoring the device, or turning off music syncing and turning it on again (though it may reproduce if you do that latter). I have no idea why this happens, but it seems that, in all cases, you see the same artists and albums in each instance of the duplicated genre.