Symfony2: Prevent duplicate in database with form

Question

I have a Parents form embedded into another form Student containing the data of the parents of a student with an association of Many-one.

StudentType.php

  //...
  ->add('responsible1', new ParentsType(),array('label' => 'Mother'))
  ->add('responsible2', new ParentsType(),array('label'=> 'Father'))

Entity Parents

 //...
 /**
 * @ORM\OneToMany(targetEntity="Student", mappedBy="$responsible1")
 * @ORM\OneToMany(targetEntity="Student", mappedBy="$responsible2")
 */
 private $students;

 //...
 public function addStudent(\Cole\BackendBundle\Entity\Student $students)
{
    $this->students[] = $students;

    return $this;
}

public function removeStudent(\Cole\BackendBundle\Entity\Student $students)
{
    $this->students->removeElement($students);
}

public function getStudents()
{
    return $this->students;
}

Entity Student

 //...
 /**
 * @ORM\ManyToOne(targetEntity="Parents", inversedBy="students", cascade={"persist"})
 */
 private $responsible1;

/**
 * @ORM\ManyToOne(targetEntity="Parents", inversedBy="students", cascade={"persist"})
 */
 private $responsible2;

 //...

public function setResponsible1($responsible1)
{
    $this->responsible1 = $responsible1;

    return $this;
}


public function getResponsible1()
{
    return $this->responsible1;
}


public function setResponsible2($responsible2)
{
    $this->responsible2 = $responsible2;

    return $this;
}

public function getResponsible2()
{
    return $this->responsible2;
}

Using this code and the following code in the driver get a new student in the Student table and two new parents in the Parent table, but in the case of then register a new student who is brother of the above, I do not know how to prevent it is believed other records with information from parents repeated after verifying that already exist in the database.

StudentController.php

/**
 * Creates a new Student entity.
 *
 */
public function createAction(Request $request)
{
    $entity = new Student();
    $form = $this->createCreateForm($entity);
    $form->handleRequest($request);

    if ($form->isValid()) {

        $em = $this->getDoctrine()->getManager();
        $em->persist($entity);
        $em->flush();


        return $this->redirect($this->generateUrl('student_show', array('id' => $entity->getId())));
    }

    return $this->render('BackendBundle:Student:new.html.twig', array(
        'entity' => $entity,
        'form'   => $form->createView(),
    ));
}

I read something to use Data Transformers or event listener as PrePersist and Preupdate, but I don't know how to use this.

Thanks in advance for your answers.