Php unusual fatal error with "name is already in use"

Question

On my local development machine (php 5.3.14) I can use a class like this:

<?php

namespace Shop\Repository;

use Shop\Entity\Day;
use Doctrine\ORM\EntityRepository;

class Product extends EntityRepository
{
    // Code
}

The class is stored in /my/src/Shop/Repository/Product.php (PSR-0 compliant). I have also a Shop\Repository\Day located at /my/src/Shop/Repository/Day.php.

However, on my staging server (php 5.3.10) I get the following error:

PHP Fatal error: Cannot use Shop\Entity\Day as Day because the name is already in use in /my/src/Shop/Repository/Product.php on line 5

I can understand the message, if I alias my Shop\Entity\Day import to DayEntity, the code works. But I cannot understand the cause of the fatal error: why does this work on php 5.3.14 (or at least, with my configuration) and not with 5.3.10 (or at least, with the server's configuration)?

I guess the problem is because in the namespace Shop\Repository there is already a Day loaded. But this has never lead to errors in my setup! What's going on?