Windows 2008 SBS duplicate forwarded emails issue?

One of my customers has a Windows 2008 SBS server. For one of the users email accounts I have set up a forward to another internal user using “Delivery Options”. The email is delivered to both the forwarding address and the users mailbox.

The issue is that whilst this works about 60% of the time (both mailboxes receiving one copy of an email), the other 40% of the time the forwarding address gets both copies (and the original recipient gets nothing).

Anyone seen this before or know how to fix it?

Regards
Mark