I also had similar experiences to those Mari reported:  Gmail users posting messages multiple times (because they didn't receive a copy), then complaining to the list owner that the list wasn't posting their messages.

I tried to implement the New_Message_ID option for my list, but discovered that it only works in Listserv version 16.0-2017a.

Since this is not installed at my institution, I threw in the towel and set all users to both ACK and REPRO.  I also set this as the default for new users. The good news:  Gmail users are happy. The bad news, non-Gmail users get a twofer (both a REPRO copy and an ACK message), which some might see as spam. Well over half our users are on Gmail, so it seemed like the way to go.

I apologized to the non-Gmail users for the additional ACK message. So far, no one has complained.

One oddity is in the web-based subscriber management page, the Acknowledgements section now displays a setting of:  [Special or obsolete setting] for users, which is less than informative.

Bill Schults
Co-owner [log in to unmask]

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