I guess it depends what the purpose is in suppressing duplicates. I
suspect people don't want to see multiple copies of the same message
in
the same folder.
Well, you don't have to suspect. I've already explained in this
discussion why I don't want to see multiple copies of the same message,
and that isn't it. :)
well, I'm not sure if I found the message that explains _why_ you don't
want to see them, but I did find a message that explains that you want
replies to list mail to end up in a list folder and not in your inbox.
I agree that it would be somewhat tricky to have recipient-side
duplicate suppression work in the way you'd like - either a message
would mysteriously disappear from your inbox and move to a list folder,
or you'd have to delay incoming mail that looked like it might have
been copied to a list. probably the former.
though I could certainly imagine a user agent being able to "hide"
messages in one folder if they appeared in another, I don't know how
much interest there would be for such a feature.