I have SS 2.9.6, OS 10.8.2 and Outlook for Mac 2011 14.3.1. I have two accounts coming in to Outlook, one is an exchange server account (work) and the other is gmail (IMAP).
Here is the issue- some of the mail that gets moved to the SS folder is duplicated there. In other words, it comes into the account as one email, but then I get two or sometimes three copies of it in my SS folder. Now this is not for all emails- but about 50% of them.
I cannot find a common condition on the ones that duplicate and the ones that do not.
When I had this set-up on Apple mail (before moving to Outlook) this did not occur.