How to reset form page after it has been successfully submitted?
That happens because you are using $form_state['rebuild'] = TRUE; which is telling Drupal to rebuild the same form, which also mean the form will be repopulated with the last values the users entered. When you then set $form_state['rebuild'] to TRUE, $form_state['redirect'] is ignored, since the user is already redirected to the page containing the form.

The code I would use is the following one.

function answers_custom_form_answer_node_form_alter(&amp;$form, &amp;$form_state) {
  if (arg(0) == 'node' &amp;&amp; is_numeric(arg(1))) {
    // See https://api.drupal.org/api/drupal/includes%21menu.inc/function/menu_get_object/7.
    $node = menu_get_object();

    if ($node-&gt;type == "question") {
      $form['#submit'][] = "answers_custom_destination";
    }
  }
}

function answers_custom_destination($form, &amp;$form_state) {
  // Since the form submission handler is added only when the user is seeing node/%node,
  // there is no need to verify that again.
  $form_state['redirect'] = FALSE;
}


As side note, since the form alter hooks and the form submission handlers share $form_state, which is passed by reference, if something needs to be shared between those functions, it can be added to $form_state. As long as the added array index doesn't conflict with an array index used by Drupal or third-party modules, all is fine.

For example, this code would use $form_state to share a value between the hook altering the form, and the form submission handler. 

function answers_custom_form_answer_node_form_alter(&amp;$form, &amp;$form_state) {
  if (arg(0) == 'node' &amp;&amp; is_numeric(arg(1))) {
    // See https://api.drupal.org/api/drupal/includes%21menu.inc/function/menu_get_object/7.
    $node = menu_get_object();

    if ($node-&gt;type == "question") {
      $form_state['answers_custom_question'] = $node;
      $form['#submit'][] = "answers_custom_destination";
    }
  }
}

function answers_custom_destination($form, &amp;$form_state) {
  // Since the form submission handler is added only when the user is seeing node/%node,
  // there is no need to verify that again.
  $form_state['redirect'] = drupal_get_path_alias('node/' . $form_state['answers_custom_question']-&gt;nid);
}


Reference


drupal_build_form()
drupal_redirect_form()