This is actually something I tried to implement originally, and the initial function became so complex that I abandoned it with the intention to go back and add it to the next release.
You can actually submit the form of users with duplicate entries in it. As each user is registered one-by-one from the top, any duplicates further down the list with the same username or email address will generate an error and not try to add it - the error shouldn't stop the form from continuing through to the end.
Of course, this does mean your Added Users feedback will have a bunch of error notices saying the email address or username is taken, but it may be an option in the meantime until I can build in the option to automatically eliminate duplicate entries.
Let me know if that works or if you have any additional ideas - I do love ideas and feedback, so thanks again!