moderated Re: Hashtag and Topic reply to overrides
But when someone hits "reply" by email from gmail (and probably other providers), groups.io should not be trying to send both the group and the individual if the hashtag is so marked, and that is what it does,
I see you found the Gmail option to have the default Reply behavior be Reply All.
If that option is chosen then the reply arrow at the top of the message does Reply All, and the buttons on the bottom are reversed (Reply All to the left of Reply). Reply All then sends the reply to both the From address (the sender) and the To address (the group) of the original message.
I don't think there's anything Groups.io can do about this Gmail behavior, except possibly to warn about it.
The only mitigation I can think of would be to silently swallow the message to the group rather than bounce it, but that would be a violation of email standards, and might well lead to more severe confusions. Modifying the original message's To address to be something other than the group posting address might help, but could also be problematic (e.g. the member might have their email filter for the group based on the To field).
That option (making Reply All the default behavior) is not unique to Gmail.