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

Start by reproducing the serialize() and unserialize() examples in the issue, comparing single and duplicated properties on readonly fields. Trace the serialization and unserialization entry points responsible for property restoration. Done means the intended duplicate-property behavior is agreed, including whether it should be deprecated or removed, and the behavior is covered by regression tests.