It's more of an issue with Marketo adding the lead to Salesforce a second time as a duplicate. It doesn't always happens, but it happens enough to cause problems with our sales group. We don't use smart lists a lot because of the complexity of our segementation (and the fact that our sales people are horrible at putting contacts on an opportunity!) so we pull lists from Salesforce, remove certain contacts and then upload it into the Marketo program. For some reason, Marketo doesn't always realize that the people on the new list were already in the database as a Salesforce contact and adds them new again.
Thanks, Jason for the correct filter! That works perfectly for me!