Symfony forms & handling Doctrine constraint violations gracefully - php

Question

I have built a form using symfony's form builder to create new entries with the doctrine entity manager, but upon creating new entries, I occasionally run into a Integrity constraint violation: 1062 Duplicate entry'. I would like to properly handle this exception using the DRY principles, what is the best way to do this gracefully?
Preferably I would have a warning show even before submitting but after submission would be acceptable also.
FlightController.php
/**
* #Route(path="/new", name="flight_new", methods={"GET", "POST"})
*/
public function new(Request $request)
{
//Create New Flight
$flight = new Flight($this->getDoctrine()->getManager());
$form = $this->createForm(FlightType::class, $flight);
$form->handleRequest($request);
if ($form->isSubmitted() && $form->isValid()) {
$flight = $form->getData();
$entityManager = $this->getDoctrine()->getManager();
$entityManager->persist($flight);
$entityManager->flush();
return $this->redirectToRoute("flight_list");
}
return $this->render('flight/new.html.twig', array('form' => $form->createView()));
}
Flight.php The constraint is a compound key:
/**
* #ORM\Entity(repositoryClass="App\Repository\FlightRepository")
* #ORM\Table(name="flight",
* uniqueConstraints={#ORM\UniqueConstraint(name="flight",columns={"ap_from","ap_to"})}
* )
*/
class Flight
{
Thanks in advance