Multiple copies of my email sent from Outlook

Outlook is randomly sending multiple copies of my email to recipients who are (understandably) very annoyed at getting up to 30 copies of the same mail.

The mail seems to get stuck in the Outbox and while it indicates it hasn’t been sent, it is actually sending it multilple times.

I had this happen in Outlook 2007 and have now upgraded to 2010 but it is still occurring.

Can anyone advise on how to get rid of this? Is it perhaps a virus?