Description:
------------
The array contains 2 identical keys.
Reproduced in all PHP 7.*.
Not reproduced in PHP 5.6.

Test script:
---------------
<?php

class A {
    private $foo = 'fooA';
    public function dump() { return get_object_vars($this); }
}

class B extends A {
    public $foo = 'fooB';
}

$b = new B();
$object_vars = $b->dump();
var_dump($object_vars);

unset($object_vars['foo']);
var_dump($object_vars);

unset($object_vars['foo']);
var_dump($object_vars);

Expected result:
----------------
The array contains unique keys only.

Actual result:
--------------
array(2) {
  ["foo"]=>
  string(4) "fooB"
  ["foo"]=>
  string(4) "fooA"
}

array(1) {
  ["foo"]=>
  string(4) "fooB"
}

array(0) {
}