Prior to Yosemite, setting the following invisible preference would cause Mail.app to show duplicate messages rather than its default behavior of hiding them:

% defaults write com.apple.mail AlwaysShowDuplicates -bool true

This no longer seems to have any effect in Yosemite (OS X 10.10). Does anyone know of an alternate method to achieve this?

  • Out of curiosity, why would you want this? – Harv Oct 22 '14 at 17:06
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    My company has a tight quota on email. Having duplicate messages with large attachments without my knowledge is detrimental to my staying under that quota. – Eric S. Oct 22 '14 at 18:36
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    Oh and BTW, there is a bug related to this: you cannot drag and drop any message that's hiding a duplicate into a new message as an attachment. Having the mails displayed individually avoids this problem. – Eric S. Oct 22 '14 at 18:38
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    Three years later and this silly bug still exists in High Sierra. So I continue to use this invisible preference, and just filed a bug with Apple. Not that other folks can access it, but it's #35594362. – Eric S. Nov 16 '17 at 20:50
defaults write com.apple.mail _AlwaysShowDuplicates -bool true

This should be working.

  • I don't know who voted this down, but this does indeed work for me on Yosemite! – Eric S. Oct 28 '14 at 13:15
  • It works on El Capitan too. – Akira Okumura Sep 15 '16 at 3:32
  • It works on macOS Sierra (10.12) too. – John Pang May 17 '17 at 17:04
  • Works also in Mojave (10.14), and makes painfully obvious how buggy Mail.app can be. – hmijail mourns resignees Dec 24 '19 at 2:25

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