Help understand pleasу

[Jekis]

All a good day, help to understand. There is a index.php page with the code. When I add text in this page, it is duplicated and others. index.php?page_id=20 index.php?page_id=18 .......

         <?php

switch($_GET["index"])
{
case"index.php":
include '10.html';
break;
}


?>
	 <?php

switch($_GET["page_id"])
{
case"20":
include '20.html';
break;
}


?>