If you have a Gmail account and are getting duplicates in the Sent folder, this is because Outlook saves a sent message in the Sent message folder and some IMAP mail providers do the same, creating two sent message every time you send a message. To stop the duplicates, you need to change the sent message settings in Outlook’s More Settings dialog or change the settings on your IMAP account.
To change the sent message settings in Outlook 2010 or Outlook 2013, go to File, Account Settings, double click on the account, then click More Settings.
In Outlook 2010, look on the Sent Folder tab for sent item handling and make your changes.
Disable Global Setting
You could disable the option to Save sent messages (in File, Options, Mail), however, this setting affects all accounts in your profile.
If you choose this option, create ‘after sending rules’ (Apply rule on messages I send) to save sent messages for other accounts in your profile.
If you have duplicates and the Outlook Connector for your Hotmail/Live account: Check the option to Save sent items in the folder with the original. It seems goofy with Outlook 2010 and separate delivery locations. (I can't repro it here, but it’s a setting worth checking – this is on File, Options, Mail, in the Save messages section.)
Published May 10, 2011. Last updated on July 8, 2014.