Just when I thought I had posted enough for today, Andreas Amann releases a updated, Leopard-friendly version of his most excellent Mail Scripts.
Mail Scripts is a collection of eleven applescripts, bundled up in a user-friendly installer, which will remove duplicate messages in Mail.app, create rules from a selected email faster, archive messages, change SMTP servers on the fly, schedule emails for delivery later and much, much more.
They are now compatible with Leopard, although Andreas adds a note of caution about two of the scripts:
When sending messages other than text/plain (i.e., HTML messages and messages with attachments) using either the “Send all Drafts” or “Schedule Delivery” scripts, Mail might fail to actually send the message and present a dialog telling that the message failed to be sent using the server “null” – re-selecting the correct SMTP server will cause the message to be sent as intended. This seems to be a bug in Mail with the application forgetting the correct SMTP server settings which hopefully will be fixed in a future system upgrade.
Mail Scripts is donation-ware and available from Andreas’ web site 
. No mail user should be without them.
Great, but where’s the “remove duplicates” script?
It’s in the package and it’s in my install. Do you not see it?
found it. wasn’t in the place that I expected it to be. : )