How do i delete music files that will not show the 'delete' slider button in iPad music player?

I have downloaded a couple of albums from iTunes, which have left duplicate dud files. These interrupt playing on the iPad, and can't seem to be deleted. I have gone to the extent of wiping my whole music library off the iPad, but the bunch of un-deletable files are still there. It first happened about 2 weeks ago, with 6 files from one album appearing with an icon on the right hand side - a circle with a square inside. I tried to get rid of them then and couldn't, so I gave up. But this morning, I downloaded another album, and it looks like the all the files off that have been duplicated with similar undeletable files. It's annoying because the music player gets stuck on one of these, it is showing that it's playing, but no sound is produced and the line on the progress bar is stuck at 0:00.