One of the mailing lists I have subscribed is delivering 2 copies to my
maildir (qmail) directory. The other mailing lists are delivering only one
copy to their maildir directories. How can I debug this mailing list
delivery alone? Can I turn debugging on for this mailing list alone?
Why not start with a careful check of the headers (those things on either side
of your nose are wonderful diagnostic aids if applied correctly; they're not
there just to make your face look pretty;-)).
Look at particularly message-id: headers and received: headers to see if you
can discern just where the duplicates originate.
It might be something as silly as you're subscribed twice, either from your
error or theirs.
procmail mailing list