Sending an email in Outlook 2003, 2007, or 2010 produces two messages in Sent Items instead of one. Frequently, the two messages will have different From addresses, but otherwise they will be identical.

The good news is that the messages aren’t really being sent twice, so it’s more of a nuisance than anything else. However, it’s still a nuisance, and eats up mailbox and disk space. The problem is caused by having multiple email accounts configured, usually Exchange plus Hotmail, Gmail, and maybe a few more. As strange as it sounds, some end users could have two different accounts using the same reply-to address. That doesn’t sound possible, but it’s probably more common than you’d believe in some larger, complex organizations. You can fix this by creating a new Mail profile for Outlook. Add the Exchange account first and make sure everything is working. Then add your other mail accounts one by one, testing the Sent Items after adding each account. If there are two accounts that use the same reply-to address, try to keep them in separate profiles or just don’t use one of them.