I think I found out why this happens. I am using an external hard drive, not sure if this is relevant, but I believe so. I went into iTunes and deleted the songs, and asked that they be deleted from the library as well. Got same message when I tried to re-import. Went on to the external hard drive, where library is located, and none of the songs had been deleted. Deleted the songs, as well as the albums folder, re-imported, no problems. Hope that helps.