Results of Array Spread Operator with explicit numeric keys in arrays

Code
<?php

$a = [1 => 1, 2 => 2];
$b = [...$a, 3 => 3, 1 => 4];

dump($b);
Result
^ array:3 [
0 => 1
1 => 4
3 => 3
]
Used Versions
PHP 8.3, Laminas MVC 3.2, Symfony 5.2, Laravel 8.28, PHPUnit 9.5, Doctrine ORM 2.8