This plugin has been deprecated! It is no longer necessary as its functionality has been integrated into WordPress. There will not be any continued support for it. Comments to this post are also now closed.
When I released the plugin, I also simultaneously made a bug report and patch for the WP core, which was quickly accepted and applied.
Reported as a bug in trac ticket #1265.
Fixed in WordPress in changeset #2561.
Prevent visitors from leaving duplicate comments/pingbacks/trackbacks to a post.