Multiple copies of the same email message in the Deliveries tab is caused by incomplete delivery setup of an email from the Message Tab using the "Set Up Delivery" action. For completing setup delivery for a message, use the "Set up Delivery" action from the Deliveries tab. 

Should multiple Admins be working on the same email message, the best practice would be to have them work on the same email message in the Deliveries tab instead of creating a new delivery from the Messages tab.