gist.php

<?php
// This is the ideal post, based on the current pointer.
$ideal = $query->posts[ $index ]->ID;
if ( in_array( $ideal, $used ) ) {
	// If it is in the array, update the index to the next.
	// What this doesn't have, is a way to go 2+...
	$index++
}

// Grab the post.
$_post = $query->posts[ $index ];

// Update the pointer.
$index++;

<?php
function get_next_valid_post( $pointer, $posts, $used ) {
    $id = $posts[ $pointer ]->ID;
    if ( ! in_array( $id, $used ) ) {
        return $pointer;
    } else {
        $pointer++;
        return get_next_valid_post( $pointer, $posts, $used );
    }
}