Prevent double ID in Firefox

This code in header.php sets double ID in cookie on single page in Firefox. All other browsers show one ID. What is wrong?!

global $wp_query;
$post_id = $wp_query->post->ID;
$cookietime = time()+3600*24*365;
$lt = '|'
if (isset($_COOKIE['tst'])) {
	$viewedstring = $_COOKIE['tst'];
	$viewedstring .= $post_id.$lt;
	setcookie('tst', $viewedstring, $cookietime, '/', '.domain.com');
}else{
	$viewedstring = $post_id.$lt;
	setcookie('tst', $viewedstring, $cookietime, '/', '.domain.com');
}