Comment 1 for bug 424195

I'm not so sure. Lots of people want the duplicate reduction this provides, but there is a price to pay because the explicitly named recipient will only get the direct copy, not the list copy.

This is a standard Mailman feature. I think the default works for most people, but there are (I'll say "advanced") users who know how to handle the duplicates perfectly well and prefer to get the list copy. We should allow the user to chose which option they want, but I think the default is correct.