I have a laptop and desktop (both running windows 7). we use exchange server 2003 and I have outlook 2010 on my desktop and outlook 2007 on my laptop.
It seems that when I have my laptop connected to my work network (physically vs. vpn), many (although not all) emails come in twice - two duplicate emails in my inbox (exactly the same content, size, time delivered, etc). They appear on my laptop and my desktop. These are both external and internal emails.
When I shut down one of the instances of outlook, the issue goes away.
I do use a number of rules for organizing my emails. but the issue occurs on emails that are processed by rules and those that are not. I also checked all my rules and they are "move" not copy rules.
The hard part is - it doesn't happen with every email, but most. And seemingly more with internal emails than external.
Again, if I am only running one instance, it's fine, but I have been using my laptop here at the office more and more (to take to meetings, etc) and so it's annoying to have to keep opening and closing outlook.