How to Fix Duplicate Emails Sent From Apple Email

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.
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