This can happen when clicking the "Back" button on your browser as you are creating shipments for a batch.
Another possibility is that your batch may have restored itself because not all of the proposed shipments for the batch were created at one time on the Proposed FBA Shipping page, therefore, leaving the remaining shipment(s) open.
Anytime you list a batch, we must refresh the batch status as Amazon requires, due to the warehouse rotation that occurs every 48 hours. When this happens, new shipments for the entire batch will get created, even if you have already created some.
To evade issues with duplicate proposed shipments, we recommend avoiding clicking the "Back" button on your browser and creating all of your shipments right away.
There is a countdown available on the Ship It page to show when the options expire, so be sure to either create or add the shipment to an existing shipment before the time runs out.
Solution
If this happens, any shipments created for the batch will need to be deleted. To comply with Amazon's Shipping Policy, you will then need to create new shipments for the item after manually closing and restoring the batch: