Showing Duplicate Messages in Apple Mail
Since Mac OS X 10.6, Apple Mail has hidden duplicate e-mails. It uses a loose definition of “duplicate,” which can consider two messages to be duplicates even if both their headers and body differ.
This has caused problems for me when dealing with mailing lists. For example, if a message is sent both to me and to the list, it’s hard to tell from what Mail shows whether I was sent a private reply or whether everyone on the list saw the message.
It can also cause problems when archiving e-mails or moving them between mail programs. Most people would expect the number of messages in a mailbox to stay constant. However, if you archive e-mails from Mail to EagleFiler, you may end up with what looks like too many messages in EagleFiler. And if you use Mail to import from EagleFiler, it may look like some of your messages were lost. In both cases, the right number of messages are there, but you can’t see that because Mail isn’t showing all of them.
Fortunately, there is a hidden preference to tell Mail not to hide the “duplicate” messages. You can just enter this command in Terminal:
defaults write com.apple.mail AlwaysShowDuplicates -bool true
Apple also says that you can adjust Mail’s preferences so that when you click on a message it shows links to the duplicates. However, this does not seem to work reliably for me, and it’s only for individual messages. It won’t help you see how many messages are really in a mailbox.
Update (2020-01-25): On macOS 10.10 and later, the command is:
defaults write com.apple.mail _AlwaysShowDuplicates -bool true
