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<!-- This will find what the ID is of the top-most parent Page, in a nested child page. For example, this page you are literally looking at is nested under -->

<?php

if ($post->post_parent) {
    $ancestors=get_post_ancestors($post->ID);
    $root=count($ancestors)-1;
    $parent = $ancestors[$root];
} else {
    $parent = $post->ID;
}

?>

<!-- $parent will be the correct ID. For example, for use with wp_list_pages. -->