Symfony2 UniqueConstraint and UniqueEntity (Integrity Violation)

I've got a little problem with a Doctrine entity for a few days now. I've defined it with a UniqueConstraint on two fields with a UniqueEntity Validation on these two fields after that. But after validating of my form with a adding of an entity already in base, no way to obtain my error message in my form.

Just an error message from Symfony :

SQLSTATE[23000]: Integrity constraint violation: 1062 Duplicate entry '339057986-00012' for key 'SIRET'

Here is my entity declaration, all seems fine to me but maybe have I forgotten or misunderstood something?

<?php

namespace Proetco\FrontBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Symfony\Bridge\Doctrine\Validator\Constraints\UniqueEntity;
use Symfony\Component\Validator\Constraints as Assert;

/**
 * Proetco\FrontBundle\Entity\Entreprise
 *
 * @ORM\Table(name="entreprise", uniqueConstraints={@ORM\UniqueConstraint(name="SIRET", columns={"SIREN", "NIC"})}) 
 * @ORM\Entity(repositoryClass="Proetco\FrontBundle\Entity\EntrepriseRepository")
 * @UniqueEntity(fields={"SIREN","NIC"}, message="Cette entreprise est déjà enregistrée")
 */
class Entreprise {

protected $Siret;

/**
 * @var integer $id
 *
 * @ORM\Column(name="id", type="integer")
 * @ORM\Id
 * @ORM\GeneratedValue(strategy="AUTO")
 */
private $id;

/**
 * @var string $SIREN
 *
 * @ORM\Column(name="SIREN", type="string", length=9)
 */
private $SIREN;

/**
 * @var string $NIC
 *
 * @ORM\Column(name="NIC", type="string", length=5)
 */
private $NIC;

/**
 * @var string $RS
 *
 * @ORM\Column(name="RS", type="string", length=45, nullable=true)
 */
private $RS;

/**
 * @var string $NCOM
 *
 * @ORM\Column(name="NCOM", type="string", length=45, nullable=true)
 */
private $NCOM;

/**
 * @var string $CPOS
 *
 * @ORM\Column(name="CPOS", type="string", length=5, nullable=true)
 */
private $CPOS;

/**
 * @var string $LCOM
 *
 * @ORM\Column(name="LCOM", type="strin