Mac Mail duplicating emails

I've always felt that the Mail app in the Mac operating system, (currently Sonoma 14.1.2), was probably the least well curated/refined of their apps. It's as if they decided it was perfect ages ago, and never bother to update it. However, over about the last year, I also have been annoyed by the fact the some emails pop up with a duplicate shortly after coming in - not all, just some - but it even happens to my own if I copy myself. Even more annoying is they tend to hang around even when opened and read, with the blue (allegedly unread) dot remaining, making it look like you have unread emails still. Anyone else having this, which is not quite the same as the OP's issue, and is there a fix..? Pete

I got the problem too, only for one of my mailboxes. I tried the delete and add again. First I thought it was working, but than the problem occurs again. This kind of Bugs are really frustrating.

VegasWineGuy wrote:

My Mac Mail app on my MacBook Pro M1 shows some but not all emails duplicated. iCloud does not show duplicates, nor does my iPhone. When I click one of the duplicates, I can forward, file, delete it, etc., but the duplicate will not load when I click on it. I cannot file or delete it until after quitting the Mail app and starting it again. I've tried reloading Ventura from safe mode, which did not fix it. Any ideas? This is very frustrating.

For problem accounts on the Mac only—

the go to way to sort things out is to delete the account(s) from the Mail.app, then add it back.

Remove email accounts in Mail on Mac

Remove email accounts in Mail on Mac - Apple Support

Add email accounts in Mail on Mac - Apple Support

Add email accounts in Mail on Mac - Apple Support

Note—

IMAP accounts stay on the server, so no local loss, when you re-sync

POP accounts download and remove from the server—therefor save your inbox/sent box if you want them

“On My Mac" is just that, local folders you created and moved mail into. So no loss

The same duplicating problem started when I "upgraded" to Sonoma. Is there an alternative to deleting accounts? I Thought I could rebuild the mailbox instead but the rebuild menu item is inactive.

This doesn't explain why it only started happening after upgrading to Ventura which I just did yesterday on my wife's computer. Never happened on Monterey or earlier.

If I open MS Outlook, everything is fine --- this is clearly another Apple bug - I don't know if it's just a display refresh issue or something deeper --- closing and reopening Apple mail fixes it (until it happens again, I suppose)

Apple's Q/A gets worse every day

Same problem with mail duplicating. Think it may be with forwarded emails…do you have any redirects to your inbox?

How do I check whether I have redirects to my inbox?

are you for example redirecting mail from say an exchange email address to your iCloud address?