Stop showing duplicates on Mail.app Yosemite

Thanks, this was driving me nuts!

I think I have the same duplicates issue.

Do I just copy paste that line into terminal?

defaults write com.apple.mail _AlwaysShowDuplicates -bool true

Yes. This will let you see duplicates, which is what I want. To not see them change true to false.

Perfect thanks!

Here is my experience with Apple Support yesterday (12/15/2014) after having all the problems listed by other people concerning duplicate emails, good mail going to Junk box, unable to delete mail, etc, etc. This is the feedback I left them. BTW, the tech advised me that no Apple employee reads this support community site. It is strictly for customer discussion, not Apple solutions. (Their words, not mine.)

My response to satisfaction survey: "Started Chat @ 11:30 and lost connection five minutes later and stayed on hold until 12:06. Chatted with Dan for 16 minutes and was transferred, on hold three minutes, then transferred again to a voice chat via phone. Great tech guy named Zachary spent 65 minutes using screen view to walk me through several changes to my mail settings, etc. He could not correct the problem and consulted a Senior Advisor for several minutes and came back to the phone and advised that the Senior Advisor suggested I change my mail settings on the webmail account, rather than on my Mac. After 12 years of excellent service by my provider, the problem developed the day I downloaded Yosemite, so I didn't like the Senior Advisor's suggestion. Zachary then had the Senior Advisor, Lois, come on the phone. She took four minutes to advise me that this was a known problem with Yosemite, and no solution was available, but engineers were working on the problem, and an update would be furnished when a solution was discovered. Lois than supplied her direct phone extension I wanted to call for progress reports in the future. So, after three hours I spoke with an Apple representative for four minutes, who already new there was a "known problem" with Yosemite mail. Might have been nice to have the other four Apple representatives advised of this "Known problem" so they didn't waste their time or mine for two and a half hours. Started Chat at 11:30 and finished at 2:08. No known solution for this known problem

Thank you thank you thank you!

This nightmare has wasted two days of my life.

Having fixed this with your code, I believe that you, Sir, are entitled to my first born son.

What a ridiculous 'feature' of mail. Why would I ever want my mail application to randomly hide important emails from me? If there WERE duplicates, well, I'll delete them myself.

I sent this feedback to Apple.

Mail (I tested on 3 macs running Yosemite & El Capitan) does not display all email messages on an IMAP server. These messages are visible by using Outlook, Thunderbird and on webmail. It seems that mail thinks some messages are duplicates (THEY AREN'T) and hides them. The code:

defaults write com.apple.mail _AlwaysShowDuplicates -bool true

Shows the so called duplicate messages.

Hiding email messages by default is ridiculous and could cause legal nightmares for customers unable to prove they sent an important message.

Please fix the default behavior to show all messages and never hide any.

If this is a problem for you, please also send feedback to apple.

https://www.apple.com/feedback/mail.html