code: public function run(): void
      {
          $schemaManager = $this->connection->getSchemaManager();
  
          // Back up the existing subscriptions
          if (!$schemaManager->tablesExist(['tl_newsletter_recipients_backup'])) {
              $this->connection->query('
                  CREATE TABLE
                      tl_newsletter_recipients_backup
                  LIKE
                      tl_newsletter_recipients
              ');
  
              $this->connection->query('
                  INSERT
                      tl_newsletter_recipients_backup
                  SELECT
                      *
                  FROM
                      tl_newsletter_recipients
              ');
          }
  
          // Find multiple subscriptions for the same channel with the same e-mail address
          $duplicates = $this->connection->query('
              SELECT
                  pid, email
              FROM
                  tl_newsletter_recipients
              GROUP BY
                  pid, email
              HAVING
                  COUNT(*) > 1
              ORDER BY
                  pid
          ');
  
          while (false !== ($duplicate = $duplicates->fetch(\PDO::FETCH_OBJ))) {
              $count = 0;
  
              // Find the oldest, active subscription preferring real subscriptions over imported ones
              $subscriptions = $this->connection->prepare("
                  SELECT
                      *
                  FROM
                      tl_newsletter_recipients
                  WHERE
                      pid = :pid AND email = :email
                  ORDER BY
                      active = '1' DESC, addedOn != '' DESC, id
              ");
  
              $subscriptions->execute(['pid' => $duplicate->pid, ':email' => $duplicate->email]);
  
              while (false !== ($subscription = $subscriptions->fetch(\PDO::FETCH_OBJ))) {
                  if (0 === $count++) {
                      continue; // keep the first subscription
                  }
  
                  $delete = $this->connection->prepare('
                      DELETE FROM
                          tl_newsletter_recipients
                      WHERE
                          id = :id
                  ');
  
                  $delete->execute(['id' => $subscription->id]);
              }
  
              $this->addMessage(sprintf('<li>%s</li>', $duplicate->email));
          }
  
          if ($this->hasMessage()) {
              $translator = $this->container->get('translator');
  
              $this->prependMessage(sprintf(
                  '<h3>%s</h3><p>%s</p><ul>',
                  $translator->trans('duplicate_subscriptions'),
                  $translator->trans('duplicate_subscriptions_begin')
              ));
  
              $this->addMessage(sprintf('</ul><p>%s</p>', $translator->trans('duplicate_subscriptions_end')));
          }
      }
docs: {@inheritdoc}