[1] If I hit reply-all to an email from a list, it replies to the list and the original poster (OP). If someone else then hits reply-all to that email, it gets sent to me, the OP, and the list. If someone else then hits reply-all to that email, it gets to sent to the second responder, me, the OP, and the list, etc. While some MDAs are smart enough to suppress duplicate delivery, a lot of them arent.
Just to follow this point up, this point seems like it might have a bigger impact if we still lived in the days where you were changed $30/MB for email. Data is basically flatrate, so sending lots of emails doesn't matter any more. However, spam and virus scanning isn't cheap. I've seen threads on mailing lists CC'd to 6 lists members and the list itself. That's another 6 passes through spam and virus scanning processes. Unless spamassassin etc are smart enough to do duplicate suppression themselves - anyone have any idea if that's the case?