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Great add-on, just wish it worked for gmail IMAP Rated 4 out of 5 stars
It takes some tweaking sometimes (i.e. changing up the search criteria) to get all of them, but this add-on is fantastic!! Having recently moved (or more aptly in the process of moving) to Gmail IMAP services, I do see it doesn't work very well in this context (which is really too bad, as I was counting on this extension to help me merge my online and old pop folders and weed out the duplicates)...
Please file a bug...
If you're having trouble with dupe removal on an IMAP account, file a bug using http://www.mozdev.org/bugs or at least email me. The extension does work with IMAP accounts including GMail.