The Mac OS X operating system includes a basic email client called Apple Mail that can connect to any email account that uses the POP3 or IMAP protocols. With older versions of Mac OS X, you may experience an issue where Apple Mail consistently duplicates messages in your "Sent" folder or mailbox subfolders. To permanently resolve the problem, reset the system file responsible for sorting email items, as it has likely become corrupted.
Quit the Apple Mail program if it is currently running or open.
Click on the "Finder" icon in the dock at the bottom of your desktop.
Go to "File" in the top menu and choose the "Find" command.
Type "MessageSorting.plist" into the search field and press "Enter" to perform a search.
Click on the "MessageSorting.plist" filename underneath the file's icon.
Add ".old" to the end of the filename, so that it changes to "MessageSorting.plist.old."
Press "Enter" to save the changes to the filename.
Restart the Apple Mail program. The application automatically creates a new system message sorting file and resolves the duplication problem.