People are not actually putting both addresses in. Their software is
doing it for them. Basically, this occurs when people are replying to
previous posts that were sent to "extropy.org" instead of "extropy.com".
The List software takes those posts under the former name, but sends
them out from the latter name. Thus, there are two addresses in the
article. A default Reply-to-All will reply to both unless the person
manually edits out the CC: header after the reply message is generated.
-- Harvey Newstrom <harv@gate.net>