If you something like:

$mform->addElement('editor', 'questiontext', get_string('questiontext', 'question'),

                 array('rows' => 15), $this->editoroptions);

$mform->setType('questiontext', PARAM_RAW);

$mform->addRule('questiontext', null, 'required', null, 'client');

$mform->addRule('questiontext', null, 'required', null, 'client');

You get a fatal (and non-sensical) error. While the above example is kinda ridiculous, it was found because a plugin wants to mark standard field as required, and we were changing the standard field to be required.

This should do one of these:

1. Be silent. (Probably just ignore the second rule)
2. Throw a dev warning.
3. Throw a sensical fatal message.

I probably vote for number 2.