Identifying duplicates is based upon two tracks have the same Musicbrainz Id, so you must match your tracks with Musicbrainz before attempting to to find duplicates.
If you are using iTunes on OSX it should be configured by default you can check this by looking at ‘Preferences/Save/iTunesAutoUpdate’. If you ‘Preferences/Save/iTunesAutoUpdate’
Run ‘Autocorrect’ (This will do the retrieve ids, autocorrect from MB etc)
Run ‘Action/Delete Duplicates’
This will find duplicates and when a duplicate is found mark the lowest quality track for Deletion, this will be indicated with a ‘D’ in the Status column
Review the tracks marked for deletion, and if you are happy really delete them using with ‘File/Save Chnages’
Run ‘Autocorrect’ should run ‘Autocorrect from Musicbrainz’ and ‘Update Tags from Discogs’, you can check this by looking at ‘Preferences/Manipulators/Autocorrecter’
Artwork can be retrieved from both Musicbrainz and Discogs
After adding artwork it should be available to the track from with iTunes.
(I have made some improvements to artowk getting in the forthcoming release of Jaikoz out before the end of the month)