Problem with duplicate $_GET['lang'] variable in URL

Hi all,
I'm working on a multilingual site for the first time. And I have problem with duplicate $_GET['lang'] variable in URL. I don't have a clue how to remove it from the URI if $_GET['lang'] is already set.

This is my url handling code:

if ( isset( $_SERVER['REQUEST_URI'] ) ) {
	$uri = $_SERVER['REQUEST_URI'];
} else {
	$uri = $_SERVER['SCRIPT_NAME'];
	if ( isset( $_SERVER['QUERY_STRING'] ) && !empty( $_SERVER['QUERY_STRING'] ) ) {
		$uri .= '?' . $_SERVER['QUERY_STRING'];
	}
}
$_SERVER['REQUEST_URI'] = $uri;

PHP:

I want let user change the language without leaving the current page.
So pleaaase help.
Thanx in advance

 

first do a debug

print_r($_GET);