Duplicated properties in serialisation payload

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.

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

N/A

Operating System

No response

首先重现 issue 中的 serialize() 和 unserialize() 示例，比较 readonly 字段上的单个属性和重复属性。跟踪负责恢复属性的序列化和反序列化入口点。完成的标准是已就重复属性的预期行为达成一致，包括是否应将其标记为 deprecated 或移除，并且该行为已由回归测试覆盖。