__set calls 2 times?

<?
    class foo
    {
        /* note: $this->____baz is not defined as var */
        public function __set($index, $value)
        {
            var_dump(debug_backtrace(0,1));
            $foo = $this->filterApply('arx', __METHOD__, array('i' => $index, 'v' => $value));
            $index = $foo['i'];
            $this->____baz = $index;
        }
    }
?>

result:
array(1) {
  [0]=>
  array(6) {
    ["file"]=>
    string(48) "/var/www/web/dev/index.php"
    ["line"]=>
    int(32)
    ["function"]=>
    string(5) "__set"
    ["class"]=>
    string(18) "ILLI\System\Object"
    ["type"]=>
    string(2) "->"
    ["args"]=>
    array(2) {
      [0]=>
      &string(3) "baz"
      [1]=>
      &string(3) "bar"
    }
  }
}
array(1) {
  [0]=>
  array(6) {
    ["file"]=>
    string(79) "/var/www/web/dev/ILLI/3.0.2/VENDOR/ILLI/System/Object.php"
    ["line"]=>
    int(84)
    ["function"]=>
    string(5) "__set"
    ["class"]=>
    string(18) "ILLI\System\Object"
    ["type"]=>
    string(2) "->"
    ["args"]=>
    array(2) {
      [0]=>
      &string(7) "____baz" <----------- WTF!
      [1]=>
      &string(34) "***ILLI\SYSTEM\FOO::FINDEX->BAZ***"
    }
  }
}


object(ILLI\System\UUID)#16 (12) {
  ["__value":"ILLI\System\UUID":private]=>
  string(36) "38355331-5271-445a-a962-74372e334979"
  ["__splHashRegisterAddress":"ILLI\System\Object":private]=>
  NULL
  ["__adapterTracesEnabled":"ILLI\System\Object":private]=>
  bool(false)
  ["__adapterTraces":"ILLI\System\Object":private]=>
  array(0) {
  }
  ["__adapters":protected]=>
  array(0) {
  }
  ["__observerTracesEnabled":"ILLI\System\Object":private]=>
  bool(false)
  ["__observerTraces":"ILLI\System\Object":private]=>
  array(0) {
  }
  ["__observers":protected]=>
  array(0) {
  }
  ["__filterTracesEnabled":"ILLI\System\Object":private]=>
  bool(false)
  ["__filterTraces":"ILLI\System\Object":private]=>
  array(1) {
    ["ILLI\System\Object::__set"]=>
    array(2) {
      ["ILLI\System\Foo::findex"]=>
      array(2) {
        [0]=>
        array(2) {
          ["i"]=>
          string(34) "***ILLI\System\Foo::findex->baz***"
          ["v"]=>
          string(34) "***ILLI\System\Foo::findex->bar***"
        }
        [1]=>
        array(2) {
          ["i"]=>
          string(38) "***ILLI\System\Foo::findex->____baz***"
          ["v"]=>
          string(65) "***ILLI\System\Foo::findex->***ILLI\SYSTEM\FOO::FINDEX->BAZ******"
        }
      }
      ["ILLI\System\Foo::toUpper"]=>
      array(2) {
        [0]=>
        array(2) {
          ["i"]=>
          string(34) "***ILLI\SYSTEM\FOO::FINDEX->BAZ***"
          ["v"]=>
          string(34) "***ILLI\SYSTEM\FOO::FINDEX->BAR***"
        }
        [1]=>
        array(2) {
          ["i"]=>
          string(38) "***ILLI\SYSTEM\FOO::FINDEX->____BAZ***"
          ["v"]=>
          string(65) "***ILLI\SYSTEM\FOO::FINDEX->***ILLI\SYSTEM\FOO::FINDEX->BAZ******"
        }
      }
    }
  }
  ["__filters":protected]=>
  array(1) {
    ["ILLI\System\Object::__set"]=>
    array(2) {
      [0]=>
      object(ILLI\System\FilterAlias)#17 (3) {
        ["__event":"ILLI\System\FilterAlias":private]=>
        string(3) "arx"
        ["__TRIGGER":"ILLI\System\FilterAlias":private]=>
        object(ILLI\System\Method)#19 (3) {
          ["__class":protected]=>
          string(18) "ILLI\System\Object"
          ["__function":protected]=>
          string(5) "__set"
          ["__method":protected]=>
          string(25) "ILLI\System\Object::__set"
        }
        ["__FILTER":"ILLI\System\FilterAlias":private]=>
        object(ILLI\System\InvokeMethod)#23 (1) {
          ["__callable":protected]=>
          object(ILLI\System\Method)#25 (3) {
            ["__class":protected]=>
            string(15) "ILLI\System\Foo"
            ["__function":protected]=>
            string(6) "findex"
            ["__method":protected]=>
            string(23) "ILLI\System\Foo::findex"
          }
        }
      }
      [1]=>
      object(ILLI\System\FilterAlias)#26 (3) {
        ["__event":"ILLI\System\FilterAlias":private]=>
        string(3) "arx"
        ["__TRIGGER":"ILLI\System\FilterAlias":private]=>
        object(ILLI\System\Method)#27 (3) {
          ["__class":protected]=>
          string(18) "ILLI\System\Object"
          ["__function":protected]=>
          string(5) "__set"
          ["__method":protected]=>
          string(25) "ILLI\System\Object::__set"
        }
        ["__FILTER":"ILLI\System\FilterAlias":private]=>
        object(ILLI\System\InvokeMethod)#28 (1) {
          ["__callable":protected]=>
          object(ILLI\System\Method)#29 (3) {
            ["__class":protected]=>
            string(15) "ILLI\System\Foo"
            ["__function":protected]=>
            string(7) "toUpper"
            ["__method":protected]=>
            string(24) "ILLI\System\Foo::toUpper"
          }
        }
      }
    }
  }
  ["____baz"]=>
  string(38) "***ILLI\SYSTEM\FOO::FINDEX->____BAZ***"
}