How to get Mail.app to display (rather than hide) duplicate messages 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?
defaults write com.apple.mail _AlwaysShowDuplicates -bool true


This should be working.