Multiple loops on parent homepage

Hi,

I’m relatively new to MultiSite. I have a client who really wants three distinct websites, but also wants a homepage that aggregates content from all three sites. Currently, I’m using this bit of code to pull posts from each site:

<?php
                        global $switched;
                        switch_to_blog(2); //switched to 2

                        // Get latest Post
                        $latest_posts = get_posts('category=-21&numberposts=4&orderby=post_name&order=DSC');
                        $cnt =0;?> 

                            <?php foreach($latest_posts as $post) : setup_postdata($post);?>

My content
                             <?php endforeach ; ?>

                    <?php restore_current_blog(); //switched back to main site ?>

The problem is that I would like to include another few posts from this site further down the page, and I’m curious if there’s a way of doing so that wouldn’t allow duplicate posts. Thanks in advance, WordPress community!