Issue #3271136 by RenatoG: Add a validation to don't accept duplicated classes

      $classes_field = $third_party_settings['block_class'];

      // Unset values that aren't classes.

      unset($classes_field['attributes']);

      unset($classes_field['remove_item']);

      unset($classes_field['add_another_item']);

      // Removed blank values.

      $classes_field = array_filter($classes_field);

      // If there are duplicated classes send a message.

      if (!empty(count($classes_field) !== count(array_unique($classes_field)))) {

        $form_state->setErrorByName('class][third_party_settings][block_class', $this->t("There are duplicated classes"));

        return FALSE;

      }

      foreach ($classes_field as $field_id => $class_field) {

        if ($field_id == 'classes' || $field_id == 'attributes' || $field_id == 'remove_item' || $field_id == 'add_another_item') {