Did you ever receive e-mail messages where the sender included themselves in the CC? Sometimes this address is exactly the same as the sender address (and you are wondering if their mail program would not be able to store sent messages), sometimes it's another address of the sender -- for example someone is writing you from a personal account and set his university account in the CC.
A small-scale non-representative empirical study revealed that most of the people cc-ing a message to themselves are professors.
When I was assigned a university mail account, I added this to my mail reader. I use Ma Gnus v0.6 in Emacs 24.2. I received the first messages and answered some. When I looked at my inbox again, I discovered that I had sent myself some messages!
I tested a bit and it turned out that when answering messages to my university account, a CC to myself was added automatically. When I answered messages to my private account, no CC to myself was added. Magical! My Gnus could distinguish my private from my professorial self.
The solution then was easy: I always use "S W" (wide reply and yank) and then maybe exclude addressees. It's less error prone than deciding whether to use "R" (reply) or "S W" for every answer and then in most of the cases where you should have pressed the "reply to all"-button will send it to one addressee only.
But then you have to include something like this in your .gnus to prevent talking to yourself:
(mapconcat 'regexp-quote '("firstname.lastname@example.org"