can't figure out how to search on first From header?

That first from line shouldn't be there. It's not a valid header because of the lack of the colon. I believe MailMate handles it by skipping the line.

The line probably became part of the email as part of some export/import cycle in which the so-called mbox format was involved (this is a format which allows multiple emails in a single file, but it's not a very good format).

That's the theoretical part. You have a practical problem, but it's not straightforward how to solve it since MailMate doesn't see this line.

MailMate has a menu item named “View ▸ Select Duplicates” which almost solves your problem. The problem is that it looks at both Message-ID (another header) and the size of the message to find duplicates. The latter won't work for your emails because of the added line.

But I can see I have a hidden preference which might help you:

defaults write com.freron.MailMate MmSelectDuplicatesBasedOnMessageIDOnly -bool YES

Be careful using that though since there is a small risk that some emails do not have unique message ids. (In most cases, that is going to be spam though.)

I hope this helps.

Thanks, this does indeed remove the problem I had with it not finding duplicates. Unfortunately now it finds the wrong copy as "the duplicate to be removed" in many cases - meaning, it selects the ones with the "good" From to remove, leaving the ???@??? ones in the mailbox. I realize this is not a problem for MailMate but it could be a problem for some other mail client.

I've found a IMAP dedup python script and I may just hack it to "do the write thing" :-)