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=wuxmojyxemesw7fe9g9cbpo3zs1jxc...@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 >
