Try "Restore Library 2 Step" to Preserve Playlists

[Eric Irwin]

On the Doug's Applescripts site you can download this set of Scripts.  
The first one will write the name of Playlists associated with a song 
in the comment field.  Then when you delete the Library file and 
reimport the files, you can run the second script, which will rebuild 
your playlists.  It does not work for Smart Playlists, but those are 
pretty easy to remake.

The other option is to just run the first script and then manually go 
through the library and find the duplicates that aren't in a playlist 
and delete them.  Not as easy, but you won't have to rebuild smart 
playlists.

Hope this helps, I find these scripts indispensable.
Eric