Recently I've been getting a slew of duplicate comments on my blog. I suspect it's mostly because the web provider for mohea.com has been unbelievably slow lately, which leads people to hit 'Post' over and over and over again until suddenly there are five identical comments.
Today I found instructions on how to modify Movable Type to block duplicate comments (defined as same name, same url, same comment text). The instructions even specified how to make it work with the MT-Blacklist plug-in that I'm using to block comment spam.