Jeremy,
It¡¯s every post by someone other than me.
I get two copies of messages. One that goes to the group and to me,
and one that goes just to the group which I am part of.
The screen-shots you sent me off-list show that the sender sent the message to both you directly, and to the group. So that's why you received it twice.
The question then becomes, "why did the sender do that?"
1) One possible answer is that the sender has been "trained" by prior experience to always use "Reply All" when replying to a mailing list.
2) Maybe the sender has their email interface set to use Reply-All by default.
3) Maybe your group has "Group and Sender" selected in the Reply To option.
Both 1 & 2 seem unlikely to be true of all members, but it might be the case if there's some common heritage in the membership of your group.
The third you've said is not true.
So I'm out of guesses.
Shal
Shal Farley