Email Variable Check
Currently if you have two user variables in the 'To' field of a notification task and both variables happen to be set to the same user, that user gets two emails from the notification task. It would be great if the notification task could check to make sure it's not sending duplicate emails to the same user. This is possible in SharePoint Designer, but is currently not possible in Nintex Workflow.
Jeff Brewer commented
I agree posted in the support forums. In my case I had the item "Created By" and a separate item field "Customer" (user type field) as the "To" recipients. With "send individually addressed notifications" checked a user will receive two emails if he or she happens to be both the "Customer" and the person who created the item. My post is here: https://community.nintex.com/thread/19990-duplicate-emails