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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?

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  • Out of curiosity, why would you want this?
    – Harv
    Oct 22, 2014 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, 2014 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, 2014 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, 2017 at 20:50

1 Answer 1

Reset to default
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defaults write com.apple.mail _AlwaysShowDuplicates -bool true

This should be working.

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  • I don't know who voted this down, but this does indeed work for me on Yosemite!
    – Eric S.
    Oct 28, 2014 at 13:15
  • It works on El Capitan too.
    – Akira Okumura
    Sep 15, 2016 at 3:32
  • It works on macOS Sierra (10.12) too.
    – John Pang
    May 17, 2017 at 17:04
  • Works also in Mojave (10.14), and makes painfully obvious how buggy Mail.app can be.
    – hmijail mourns resignees
    Dec 24, 2019 at 2:25

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