Symfony Routing bug?

test.php

<?php

require __DIR__ . '/vendor/autoload.php';

use Symfony\Component\Routing\Route;
use Symfony\Component\Routing\RouteCollection;

$masterRoutes = new RouteCollection();

$routesA = new RouteCollection();
$routesA->add('hello', new Route('/hello', array('controller' => 'foo')));
$routesA->addPrefix('/a/');

$routesB = new RouteCollection();
$routesB->add('hello', new Route('/hello', array('controller' => 'foo')));
$routesB->addPrefix('/b/');

$masterRoutes->addCollection($routesA);
$masterRoutes->addCollection($routesB);

var_dump($masterRoutes->all());exit;

<?php

require __DIR__ . '/vendor/autoload.php';

use Symfony\Component\Routing\Route;
use Symfony\Component\Routing\RouteCollection;

$masterRoutes = new RouteCollection();

$routesA = new RouteCollection();
$routesA->add('helloa', new Route('/hello', array('controller' => 'foo')));
$routesA->addPrefix('/a/');

$routesB = new RouteCollection();
$routesB->add('hellob', new Route('/hello', array('controller' => 'foo')));
$routesB->addPrefix('/b/');

$masterRoutes->addCollection($routesA);
$masterRoutes->addCollection($routesB);

var_dump($masterRoutes->all());exit;