Delete large numbers of duplicate emails from Mail.app

This workaround, using Thunderbird, allowed me to successfully remove 30,000 duplicate emails (from a collection of about 80,000 emails) in OS X's Mail.app. I spent a lot of time searching this question, and this is the only solution I found that worked.

My mail.app emails got rather out of hand; I won't bore you with how. I had most emails at least twice, and some up to five times. I tried Andreas Amann's Mail Script for this, but, even though it was working OK, it only found about 200 duplicates in three hours, and was cooking my CPU at about 95%. There was no way that this could go on, so I cancelled the process. (Thanks anyway, AA.)

I looked at importing into Entourage, because there are some scripts to eliminate duplicates from there, but for reasons I shall not bore you with here, this proved a dead end.

The solution turned out to be the amazing add-on for Mozilla's Thunderbird called ...