Thanks to you all for giving this problem such careful thought. I wonder if Shal's double fault theory might explain it - that is, a redirect rather than forward, followed by a Reply All rather than a Reply. I don't know what email application my forwarder uses, but if Shal is really able and willing to test the theory with his Eudora-using contact, it might shed some light on this. My "offending" replier definitely didn't click on the groups.io Reply to Sender link, but it's possible they clicked on their email program's Reply All button (though I can't imagine why).
Just to clarify (Bruce) - although the question I asked back in January probably looks similar, the circumstances were quite different. I'm probably not very good at describing these things. And also just to clarify: I don't "manage two groups with members in common all discussing the same topics". One group is more than enough. The "other" group I was referring to in January was not mine, and the overlap of members probably only amounts to 3-4.?
BarryS
