Array construct/destruct should warn at least for duplicate keys

Description

$arr = ['4', 'x' => 5];

// construct with duplicate keys
var_dump(['x' => 4, 'x' => 6]);
var_dump([2 => 4, 2.0 => 6]);

// spread operator leading to duplicate keys
var_dump(['x' => 4, ...$arr]);

// destruct with duplicate keys (it is safe to do, but if same key should be assigned to another variable too, it can be done after the destruct)
['x' => $a, 'x' => $b] = ['x' => 10];
var_dump([$a, $b]);

https://3v4l.org/NTFPM

all cases should emit at least warning if not an error

首先复现 issue 中关于重复键构造、spread 和解构的示例，包括链接的 3v4l.org 案例。跟踪 PHP 解释器对数组键的处理，并确定相关的测试或入口点；完成的标准是列出的每个重复键案例至少发出警告或错误。