I had this problem too.
Are the duplicate songs from the apple itunes store?
What was weird on my iphone, once I deleted one song, and tried to play the 2nd song. There was no sound from the song!
I ended up deleting all the songs I got from the itunes store and redownloaded them again from the iphone (from the cloud) and now everything works properly.
This is a major bug in IOS 5.1!
I have this problem as well, but i dont know how to fix it!
If i delete the songs out of my itunes library and then sync with an empty library will they be lost or will i be able to re-download them from my itunes account?
(sorry if thats confusing!)
Or can i delete them directly off my phone?
I tried deleting my library and then redownloading them from my phone but it didnt work either.
I have the same problem, seems to occur when the song is sync'd and downloaded from the itunes store at the same time...i.e. when you purchase a song from iTunes while the ipod/iphone is plugged into the computer - the conflict occurs between the sync and the part downloaded songs.
The duplicate songs have the automatic download symbol next to them (similar to the sample play symbol) on iTunes, but you cannot activate the radio button to properly download the song.
I have tried deleting my entire music library from my iphone; all songs are removed except for these duplicates and they are not able to be manually deleted, all other songs can be manually deleted.
I can try restoring the iPhone but I have a few Apps that I cannot extract the data (i.e. password manager) that I do not want to lose.