This is a Greasemonkey script and favelet that will remove duplicate listings on del.icio.us.
If run as a Greasmonkey user script, it will invoke automatically and remove any duplicates it finds. As a favelet, you'll need to manually invoke it, though you will have the option of showing the duplicates that have been removed.
As a Greasemonkey Script: Right click this link and select "Install User Script"
As a Favelet, right click this link and select "Bookmark this link" del.icio.us duplicate filter