To remove the duplicate e-mail address check in Joomla 2.5 - though beware of password resetting functions.
Edit file: libraries/joomla/database/table/user.php
Comment out the lines below:
Line: 238
if ($xid && $xid != intval($this->id))
{
// XXX ALLOW DUPLICATE EMAIL ADDRESSES -
// $this->setError(JText::_('JLIB_DATABASE_ERROR_EMAIL_INUSE'));
// return false;
}