According to RealNetworks, the average iTunes library holds 5,409 songs (I only have 2689??!) and depending on the various sources you may have acquired your music from (unless you’re a complete iTunes fanboy) its likely many of them will be missing artwork, and possible that some of them are duplicates.
Well more than possible as far as Real is concerned as they’ve just released Rinse, a new product that organises and cleans your music collection. Rinse automatically looks up songs in the widely-acclaimed Gracenote® music database, downloads the correct details for each song, and applies them in your library. With more than 8 million albums and 100 million tracks in its catalogue, Rinse users have access to the largest and most up-to-date database to fix their music libraries.
As with any mass-update software, I’d strongly recommend you backup your library fist in case Rinse does some damage along with all that organising. That’s assuming you have sufficient space to backup your 5,409 songs somewhere else first!