Only warn when importing duplicate emails

This is related to issue https://www.drupal.org/node/1283710 but I just wanted to share the patch we were using. Unlike that issue we don't do anything with role assignment, so this small change doesn't effect that.

We have an organization with a large list of emails to pre-approve and that list, when modified, is provided in full. Importing the list on the "Manage E-Mails" page will always result in an error as most of the addresses are likely already in the system, which causes a headache for someone to separate out the new emails from the existing ones.

We've modified the form validation function to just display a warning and remove the duplicate email from the form submission. The patch is attached, but I'm not sure if there's wider repercussions to doing this as per the role assignment end of things.