Added some extra checks to ensure the user is not already in the organisation they are attempting to join

locale/en_US/messages.php

@@ -128,6 +128,7 @@ return [
             'REJECT'                    => 'Reject request to join',
         ],
         'MEMBER' => [
+            'ALREADY_EXISTS'            => 'User <strong>{{user_name}}</strong> is already a member of organisation <strong>{{name}}</strong>',
             'NOT_FOUND'                 => 'User <strong>{{user_name}}</strong> is not a member of organisation <strong>{{name}}</strong>',
             'NOT_AN_ADMIN'              => 'User <strong>{{user_name}}</strong> is not an administrator of organisation <strong>{{name}}</strong>',
             'ALREADY_AN_ADMIN'          => 'User <strong>{{user_name}}</strong> is already an administrator of organisation <strong>{{name}}</strong>',