Unicité d'une ressource impossible (je crée des doublons)

// src/AppBundle/Controller/PlaceController.php
    /**
     * @Rest\View(statusCode=Response::HTTP_CREATED)
     * @Rest\Post("/places")
     */
    public function postPlacesAction(Request $request)
    {
        $place = new Place();
        $form = $this->createForm(PlaceType::class, $place);

        $form->submit($request->request->all()); // validation des données

        if ($form->isValid()) {
            $em = $this->get('doctrine.orm.entity_manager');
            $em->persist($place);
            $em->flush();
            return $place;
        } else {
            return $form;
        }
    }

// src/AppBundle/Form/Type/PlaceType.php
namespace AppBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\FormBuilderInterface;
use Symfony\Component\OptionsResolver\OptionsResolver;

class PlaceType extends AbstractType
{

    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        $builder->add('name');
        $builder->add('address');
    }

    public function configureOptions(OptionsResolver $resolver)
    {
        $resolver->setDefaults([
            'data_class' => 'AppBundle\Entity\Place',
            // Cross-Site Request Forgery désactivée : pas de session utilisée (API stateless)
            'csrf_protection' => false
        ]);
    }

}

// src/AppBundle/Entity/Place.php
namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;

/**
 * @ORM\Entity()
 * @ORM\Table(name="places",
 *      uniqueConstraints={@ORM\UniqueConstraint(name="places_name_unique",columns={"name"})}
 * )
 */
class Place
{
    // ...
}

// app/config/validation.yml
AppBundle\Entity\Place:
  constraints: 
    - Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity: name
  properties:
    name:
      - NotBlank: ~
      - Type: string
    address:
      - NotBlank: ~
      - Type: string