First, thanks to everyone who came in on this with suggestions.
What I was attempting to do was match emails that have the same subject
line. In each case the message-id would be different so I couldn't use
that.
After playing around with each suggestion I managed to edit the msgdups
script that Kevin Cosgrove pointed me to so that it would look for
duplicate subect lines.
I've only run my edited msgdups.pl -rmm once but it seems to do what I
want.
In message
<CAFR-7auMg0ehC+8m=WuxmOJYxEMEsW7Fe9g9Cbpo3zs1Jxc7CA@mail.gmail.com>, Kevin
Cosgrove writes:
--000000000000159a8205a4d6361f
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
I wonder if these old [n]mh FAQ things would help? I remember using
mhfinddup a LONG time ago.
https://www.mhonarc.org/archive/html/nmh-workers/2008-09/msg00004.html
https://lists.gnu.org/archive/html/nmh-workers/2008-09/msg00001.html
https://github.com/dscho/nmh/blob/master/docs/FAQ