Bring back the duplicate customer warning message

Problem

On the old user management view, when the customer had an Atlassian account and a porta-only account with the same email. Jira used to show a warning message asking the site-admin to migrate the account to an Atlassian account and merge both accounts. This is not gone now with the new User Management look.

This brings problems to JSD projects, where the agent cannot receive notifications or access to certain pages.

Suggested Resolution

Have some kind of message, showing the site-admins that there are duplicate accounts. Add "duplicate accounts" on the filtered by drop-down.