Users are reporting duplicate copies of messages in their sent folder:
Our accounts consistently get double messages in the sent folder. The recipient only receives one copy, but there are always two in the sent message folder.
The cause of this phenomenon depends on the type of email account involved.
In the case of IMAP accounts, Outlook 2010 saves a sent message in the Sent message folder and your IMAP mail provider may also save a copy. The result is two messages in the Sent Items folder every time you send a message. This seems to be most common with Gmail accounts but other IMAP accounts may be affected as well.
Change the IMAP Account's Sent Options
First, check your settings for saving Sent items. Open Account Settings (it's on the File menu in Outlook 2010) - double click on your IMAP account then look at the settings on the Sent Items tab. Choose the option for Do not save copies of sent items and send a test message to see if the problem persists.
Change Outlook's Global Options
If you have only one email account in your profile, you could use the option to not save sent messages. This is in File, Options, Mail, in the "Save messages" section, uncheck the option to "Save copies of messages in the Sent folder". Note that this setting will apply to all accounts - if you have more than one email account configured, you will not save sent items for any account.
For this reason, you need to be careful using the global setting in File, Options as not all mail servers will save a sent item for you when sending. It's better to disable the option to save sent items on the More Settings, Sent folder tab.
If you have an Hotmail Live account, one thing that is reported to cause duplicates in MSN (and possibly other accounts) is the option to Save sent items in the folder with the original. This is on File, Options, Mail, in the Save messages section.
Scanning outgoing mail with an antivirus scanner may result in duplicates.