Users Same Email Plugin for Joomla!
This plugin for Joomla! does simply what its name suggests: It allows multiple users to register using the same email.
By default, Joomla user management will not allow different users to register or change their email to an address alredy used by another user. Really, there is not real need to implement this policy, and under some situations, you might want the opposite to be enforced. For example, one same user holding differents accounts, different users from same household, for testing purposes, etc.
You just only have to install and activate this plugin, and your users will be able to provide any email they wish, regardless of any other user using it.