Reader Susan Swartz has tired of reading the same messages over and over. She writes:
What's the current best way to find duplicate mail messages?
Great question. Let me rephrase it so that the answer is more helpful:
What's the current best way to get rid of duplicate mail messages?
I suggest this rephrasing because, honestly, at the end of the process you want to delete Mail's duplicate messages. So let's just cut to the chase.
I've found that cutting is best achieved by downloading, installing, and running JollyRoger's free Remove Duplicate Messages AppleScript (direct download link). It works this way.
First, make sure that your email is backed up (as part of an overall Mac backup). The following process shouldn't cause any harm but if something goes wrong, you can fall back on your backup.
After downloading the archive, open it. Hold down the Option key, click the Finder's Go menu, and choose Library. Within the Library window that appears, locate the Scripts folder and drag the Remove Duplicate Messages script into it.
Launch Mail and select the mailbox you'd like to remove the duplicates from. Select all of the messages in that mailbox; then, from Mail's AppleScript menu, choose Remove Duplicate Messages. (If you don't see an AppleScript menu, launch AppleScript Editor--found in /Applications/Utilities--open its preferences, and from the General tab enable the Show Script Menu in Menu Bar option.)
A Remove Duplicate Messages folder will be created on the desktop. Within it you'll find a Read Me document, as well as the Remove Duplicate Messages.log file. If the AppleScript finds any duplicate messages, it will remove them from Mail and copy them into a Removed Duplicates folder within the Remove Duplicate Messages folder on the desktop. When the script is finished, you'll see a window that tells you how many duplicates (if any) it removed. Repeat this process for other mailboxes.
If you like, you can preview these message to ensure that they really are duplicates--and then toss them out. You can also double-click the log file to view its contents in the Console application.