Hello Nina,
This might be an issue with a lead that was deleted, and then added as a contact in Salesforce - just because it is deleted in Salesforce, doesn't mean it is automatically deleted in Marketo, which is when duplicates can be created. You will may want to include 'and not possible duplicate' by email addresss.
Make sure you consider if these leads should be assigned to a certain person or queue by a person or assignment rules as well; the leads can only be auto-assigned once.
I hope this was helpful!