Use the plugin: http://wordpress.org/plugins/allow-multiple-accounts/
Seriously, this is what we do, officially, when a feature a small group of people want is not available in core WP.
https://core.trac.wordpress.org/ticket/9563 was the ticket, 5 years ago, where this was enforced for sanity and safety. On an open registration site, having this not enforced would be brutal. On a closed site, well, we have the plugin. Which by the way was written by one of the core devs :)