[php-src] Issue #22599: Duplicated properties in serialisation payload

Issue: https://github.com/php/php-src/issues/22599
Author: NickSdot

### Description

Opening this as a reminder for the below.

@TimWolla: https://github.com/php/php-src/pull/22588#discussion_r3523424963
> It is legal for properties to appear twice in serialization payload?! This probably should be deprecated and killed.

```php
class TestDefault {
    public readonly int $prop = 1;
}

var_dump($s = serialize(new TestDefault));
var_dump(unserialize($s));

// Unserialization still bypasses normal readonly assignment semantics.
var_dump(unserialize("O:11:\"TestDefault\":0:{}"));
var_dump(unserialize("O:11:\"TestDefault\":1:{s:4:\"prop\";i:2;}"));
var_dump(unserialize("O:11:\"TestDefault\":2:{s:4:\"prop\";i:2;s:4:\"prop\";i:3;}"));

string(38) "O:11:"TestDefault":1:{s:4:"prop";i:1;}"
object(TestDefault)#1 (1) {
  ["prop"]=>
  int(1)
}
object(TestDefault)#1 (1) {
  ["prop"]=>
  int(1)
}
object(TestDefault)#1 (1) {
  ["prop"]=>
  int(2)
}
object(TestDefault)#1 (1) {
  ["prop"]=>
  int(3)
}
```

### PHP Version

```plain
N/A
```

### Operating System

_No response_