Prevent a user to create two identical items in a structure field

The CMS Questions
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@texnixe I’m trying to implement a similar function to the panel by following the function you wrote about here: Structure field update
But I can’t get i to work.

My function looks like this:

function addToStructure($pageURI, $field, $data = array()) {
  kirby()->impersonate('kirby');
  $fieldData = page($pageURI)->$field()->yaml();
  $fieldData[] = $data;
  $fieldData = yaml::encode($fieldData);
  try {
    page($pageURI)->update(array(
      $field => $fieldData
      )
    );
    return true;
  } catch(Exception $e) {
    return $e->getMessage();
  }
}

And my hook like this:

'page.update:before' => function ($page, $values, $strings) {

    if ($page == page('tag-manager')):
      $globalTags = A::pluck($page->global_tags()->yaml(), 'tag');
      $unique = array_unique($globalTags);

      if (count($globalTags) !== count($unique)):
        // There are duplicate entries
        addToStructure($pageURI = 'tag-manager', $field = 'global_tags', $data = $unique);
      endif;
    endif;
  }

And the blueprint with the structure field looks like this:

tags_section:
  type: fields
  fields:
    global_tags:
      label: Globala taggar
      type: structure
      sortBy: tag asc
      fields:
        tag:
          label: Tag
          type: text

If I enter two identical values to the structure field from the panel and hit save, the data is just saved, and the duplicates are still there. I’m using Kirby v3.2.3

What am I missing?