WP function duplicating body content

Question

Add a new filter expression immediately after the <body> tag. Like the following

<body <?php body_class(); ?>>
<?php echo apply_filters("immediately_after_body","");?>

Now in your functions.php add the filter hook like this

add_filter("immediately_after_body","my_function");
function my_function(){
    //do your calculation
    return "<h1>output from Filter Hook</h1>";
}

You can see the output immediately after <body> tag. Use filters, they were built for a purpose 🙂