It's a very unwanted action when in our system you will have duplicated email addresses. Specially if you have enabled email notifications on board.
However it's still possible to disable... In
bb_func_regusr.php try to remove the part of the condition which says
and !db_simpleSelect(0,$Tu,$dbUserId,$caseComp."({$dbUserSheme['user_email'][1]})",'=',strtolower(${$dbUserSheme['user_email '][2]}))
In
bb_func_editprf.php try to remove the code which says
if ($rowp=db_simpleSelect(0,$Tu,$dbUserId,$caseComp."({$dbUserSheme['user_email'][1]})",'=',strtolower(${$dbUserSheme['user _email'][1]}),'','',$dbUserId,'!=',$user_id) or (strtolower(${$dbUserSheme['user_email'][1]})==strtolower($admin_email) and $user_id!=1)) $correct=4;