Sorry about that. I'm migrating over from reddit, and I expected the link submission form to warn me if I submit a duplicate (as it does on reddit and digg). I'll search first before I post, going forward.
Just FYI, the link submission form will warn you, but only if it is an exact match. Since the previous posts didn't have "chapters" at the end of the url, it looked unique. Here's a handy search tool: http://searchyc.com/
It's not that huge a deal; at least it's a decent link.