Hi Trish,
I can see you have also posted a discussion over here
http://community.marketo.com/MarketoDiscussionDetail?id=90650000000PjgmAAC which means you seem to have found the answer.
For the benefit of everyone else out there, and without getting into a debate over this as best practice!, the way to accomplish this is to add leads to lead queues in SFDC using the Sync to SFDC flow step.
Once the lead goes into the CRM, if the person is actually a SFDC Contact, Marketo will create a duplicate lead and add the new lead to the lead queue, while leaving the existing Contact alone.