How can I merge two Apple Mail databases?

A few years back I got a Mac Pro desktop to go alongside my MacBook Pro. I cloned the MacBook Pro user to the Mac Pro so everything would be the same. I've been using ChronoSync to make sure all the documents are the same on each machine.

However, regarding Apple Mail, cloning my account was big mistake because my Apple Mail rules were set up to "Move" certain messages to Mail folders that are "On My Mac". I did not realize that to "move" a message would involve deleting it from the IMAP server and only storing it on my local machine!

So now I have a situation where half of my mail is on the Mac Pro, and half of it is on the laptop. What I want to do is merge all the mail from the laptop into the desktop's Mail program so that any messages that were on one, but not the other, will be added in, but no duplicates will be created.

I've not been able to find any third-party software that can accomplish this nightmarish task. I also don't want to import the messages only to find out I've created duplicates of hundreds of thousands of messages, with no way to delete those duplicates.

Can anyone think of a good way to accomplish this goal? I'm open to any options. Maybe I could import the mail from both Apple Mails into a third-party mail program that has more robust tools for finding and removing duplicates, etc.? Then possibly migrate it back into Apple Mail post-merge?

I would appreciate any advice. I realize there may be multiple valid answers to this so I will try to upvote all viable answers and checkmark the most efficient solution.