When you ask Marketo to add a Salesforce contact to a lead queue, Marketo creates a duplicate lead in Salesforce and adds that to the queue instead.
The reason why Marketo creates a duplicate is because Salesforce queues can only have leads in them, not contacts. If you try to add a contact to a queue, Marketo can either do nothing or create a duplicate lead and add that to the queue. We opted for the duplicate.
If you want to prevent this behavior in your campaigns, use a filter on "SFDC Type is Lead" to limit the action to only leads