Outlook 2010 - Sending emails twice

When one of our user's email from Outlook 2010 (any workstation), it duplicates the sent item and the recipient receives it twice.

The 1st email sends fine with all of the content, but the second only keeps the subject line and has no content in the body of the email.

The sent items appear twice in the users Sent Items folder. I have tested sending using the Outlook WebApp and it doesn't duplicate the email. The user has no client rules setup either.

Thanks in advance,

Using Exchange 2010 SP1 with SMTP