By default, all shipments from your Unleashed store will be processed by Shiptheory. 

Now, if you want Shiptheory to ignore shipments that you've previously completed, you have to enable the below option within the Advanced Settings of your Unleashed integration.

You will then see previously completed shipments as 'Not Ready' on your dashboard.

More about statuses in shiptheory 

Testing and sending your first unleashed shipment