Sorry about the duplicate posts. But it illustrates the idea that computers are really stupid.
Here is a guess at what happened. I wrote this response quickly this morning on the way to work. In unix, you can read mail and respond to mail directly off the command line (the really old and almost original `mail' facility) and then shutdown the system before the sendmail bot has been notified by the cron daemon(?) that the message went out. So, you come home, eat dinner, boot up, and boom, the sendmail bot is awake again, is notified some message didn't go out, and so the idiot bot sends it again some ten hours later. The time stamps in the header of both messages ~9:10 and ~20:50 should confirm this theory.