Duplicate Messages when sent to multiple recipients in Outlook

Every user on our mail server (whether they are using Outlook 2000 or Outlook 2003 or Outlook Express) – will recieve duplicate emails when that email is sent to multiple recipients.

e.g. If i recieve a message which is also sent to 2 of my colleagues. We will each recieve 3 copies of that email. (1 for each recipient)

Any idea how i can fix this?

Thanks