FOS\UserBundle\Entity\UserManager::validateUnique PHP Method

    public function validateUnique($value, Constraint $constraint)
    {
        $fields = array_map('trim', explode(',', $constraint->property));
        $users = $this->findConflictualUsers($value, $fields);
        // there is no conflictual user
        if (empty($users)) {
            return true;
        }
        // there is no conflictual user which is not the same as the value
        if ($this->anyIsUser($value, $users)) {
            return true;
        }
        return false;
    }