Do you have iTunes copying items to the library when adding? (it's a setting in preferences).
It happened to me too, happens time to time I guess, but sucks when you drag a LOT of songs in and it does that.
What I did was deleted every song in the library. I still had the originals in the folder I kept them in (I used to organize my music all in 1 folder). From there I simply re-drag that folder in, and let it re-copy everything over again, and it did it properly that time.
If you have deleted the folder you dragged over, and have it automatically copying music to the iTunes folder, right click one of the duplicates, and select "show in finder". If the actual song in the finder is just 1 (no duplicate files), then the problem is the iTunes library file, which can be fixed in a similar way. Just physically remove all the songs from the iTunes folder in Finder, put them in a new folder temporarily. Once that is done, highlight everything in iTunes and delete, then drag that folder you put everything in into the iTunes window. It should re-add everything, create new artist/album folders, and creating a new library file.