non overlaping choice lists in symfony2

Using Symfony 2.2.4.

I have a form with two choice lists(multiple,non expanded) showing the same elements(entities of a class). I need to throw an error(onsubmit) when the user selects the same element from both choice lists. Is there a way to validate this selection that does not need to traverse both lists checking each element, you know, like an automatic/built in validation.

I need to catch the error and bind it to one of the choice lists in a way that allows me to show it as any other error, meaning through form_errors(form).

Any tips appreciated.

the easiest way is to add a listener in the buildForm of the AbstractType class, here an example

    $builder->addEventListener(
        FormEvents::POST_SUBMIT,
        function (FormEvent $event) {
            $form = $event->getForm();
            $coll1 = $form['field1']->getData();
            $coll2 = $form['field2']->getData();
            $ids1 = $coll1->map(function($entity) { return $entity->getId(); })->toArray();
            $ids2 = $coll1->map(function($entity) { return $entity->getId(); })->toArray();
            $intersect = array_intersect($ids1, $ids2);
            if (!empty($intersect)) {
                $form['field1']->addError(
                    new FormError('here the error')
                );
            }
        }
    );

Note that I have not tested the intersection of the collections but I hope the meaning is clear

Another (a bit hardest) way is to create a custom validation constraint

here the cookbook from symfony docs