Module to test unique items are being created in advancedqueue in Drupal.
Make sure you have patched advancedqueue with https://www.drupal.org/node/2918866.
Run drush queue-items or drush queue-items --num-items=10 to queue a large number ot items at once.
Run drush queue-one-item subsequently, to try to queue the same item.
Note: in order to clean up the default-queue you can run
drush ev '\Drupal\advancedqueue\Entity\Queue::load("default")->getBackend()->deleteQueue();'