How to replace songs with new versions w/artwork?

The only time CD cover art is important is when I use my iPod. I prefer seeing the cover as each song plays, rather than the generic icon. It doesn't matter when I play songs in iTunes because I just list the songs by title and click play. To solve the iPod problem, I launched iTunes' "Get Album Artwork" function. I found art for most of the recordings where it was missing. I checked the results in "View as Cover Flow," then repaired a few still unmatched by downloading the cover art to the folder where the mp3s are located and using the iTunes "Get Info" function to add the art to the song. Then I selected all the songs in iTunes I had grouped for the iPod and dragged them over the iPod music symbol under Devices on the left, thinking the repaired songs would overwrite the previous versions.

Wrong. Instead I got duplicates of numerous songs. Worse, some of the repaired versions didn't seem to make it to the iPod with cover art intact. I'm having to delete the dupe (after making sure both have no art), then click info and manually add cover art to the iPod versions all over again. Can someone tell where I might have gone wrong here?