There is an effort to support authentication via email addresses, primarily targeted at Web applications. The SSSD ticket is https://fedorahosted.org/sssd/ticket/2856.
To avoid locking out users when such functionality is enabled and logon attempts are run against all users with the matching email address, it'd be good to be able to enforce the mail attribute to be unique. It likely should be an optional flag which can be turned on.
Before we decide(if) to go that way, it would be good to consider:
Unique email addresses are required for user portal effort, too.
Metadata Update from @adelton: - Issue assigned to someone - Issue set to the milestone: Future Releases
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