element multiple cannot be used in the child form

This is probably an issue of merging the configuration, I've found issues (values were duplicated on save) when doing blocks and using element multiple.

UsingNestedArray::mergeDeepArray like so, fixes the issue:

  /**
   * {@inheritdoc}
   */
  public function setConfiguration(array $configuration) {
    $this->configuration = NestedArray::mergeDeepArray(
      [$this->baseConfigurationDefaults(),
      $this->defaultConfiguration(),
      $configuration],
      TRUE
    );

Related to 📌 Create a trait and base class to implement \Drupal\Component\Plugin\ConfigurableInterface Needs work