Unfortunately, this script does not work properly for me. It finds duplicate messages OK, but it does not also remove those duplicate messages from the selected Apple Mail folder, which makes the script useless.
Case in point: I ran the Remove Duplicates script on a folder which contains 3,301 messages. The script found 201 duplicates, and it removed them to the archive folder. I sent those duplicates to the trash. I rebuilt the source mailbox. The source mailbox again had 3,301 messages.
So, I ran the script again on the same source folder. It again found 201 duplicates, and it removed them. The number of messages did not decrement, as I would have expected it to do. I still have the same number of messages in the source folder as when I started.
I ran the script a third time. This time, I selected the option to review each duplicated message before choosing to leave it in the source folder or send it to the archive folder. It again found those same 201 duplicate messages; I again sent all of the duplicate messages to the archive folder; the script again failed to remove those duplicate messages from the source folder in Apple Mail.
I reported this to the developer, but he did not reply.
My low rating is based on the fact that the script does not perform as intended. It finds duplicates, but it does not remove them.
Whether this is a failure of the script or a bug in Apple Mail is a moot issue. A previous Apple Script, developed by Andreas Amann, worked brilliantly before Mac OS X 10.7 (Lion) was developed. But, on his website and through correspondence with him, he points out that Apple introduced a bug in Apple Mail which prevents an Apple Script from running correctly in Apple Mail. He filed bug reports with Apple, but those bugs have not been fixed. Accordingly, he had to abandon further development of his very useful scripts. If you try to run his script on Mavericks, for instance, you will encounter an error that prevents it from getting started. While Jolly Roger's script can get started and it can apparently find duplicate messages, my tests inform me that it can not successfully finish the job of removing those messages, which renders it ineffective.