In depth on the duplicates problem. More info.

I've got the same duplicates problem a lot of people have. One song with one source will have two entries in the library. Furthermore, some of the duplicates just showed up while others appeared after editing the metadata. If I click on the duplicates of the latter reason both song entries get highlighted (meaning I can't delete one without deleting the other) and if you change the data in one, the other changes with it. The ones that appeared without me messing with the metadata are separately clickable and can be deleted but I have to do it manually. The exorcist add-on only recognized about 15-20 duplicates out of my entire library and deleted them but there were still dozens more that I had to do myself.
Finally I had gotten rid of all the duplicates I could (not counting the edited metadata ones because I still wanted the songs) and tried to sync the songs to my ipod. I noticed that below the status bar it said, "Copying: (song name)" and started naming off random songs as it was syncing. Sure enough, when I checked some of the songs on my ipod that were said to be copied, a playable duplicate was on there but when I looked for those songs on my songbird library none of them were there. I re-synced and got the same result (same songs copied). I remedied this problem by canceling the sync right as it started to copy files and the duplicates disappeared from the ipod at least.
Lastly, the duplicates on both the ipod and the songbird library are from m4a files and mp3's so it doesn't seem like there's any sort of a pattern (by source) as to which ones get duplicated.