Appending random post links to single post?

You could try this plugin:
https://rmarsh.com/plugins/similar-posts/

Is there any way to do it directly in the code? I tried to add a loop to the end of a post template and also linking to a copy of, say, index.php through the php include function. Neither worked. Rather, the post page displayed either one duplicate or many duplicates of the current post. Any ideas?