[Mailman-Users] users getting mail twice

[Christopher P. Lindsey]

> What do I need to do to remedy this?

There are two possibilities that I can think of:

   1.  Do it on a per user basis (most traditional method, and really the
       only right one based on existing SMTP protocols).  If you use 
       procmail as your local delivery agent (LDA), you can use a formail
       recipe to keep track of Message-Id: headers and remove duplicates.

   2.  If your MTA supports filtering (such as sendmail 8.11.0's libmilter
       filtering API) then you can pump your mail through a Perl script
       at the MTA stage and remove duplicates there.

       This would be a pain in the butt and pretty hard to program.

The reason that this really isn't Mailman's problem is that each one of
those addresses is considered independent of each other.  If the mailing
list and the Cc: address aren't on the same machine, how does Mailman 
know that the user received his Cc: already?  This configuration assumes
that the other mail will always get there -- there are no network outages,
no blacklists, no misconfigured MTAs...

But I should also say that this request isn't really unusual.  We've gone
through this at NCSA with umbrella lists where people are subscribed to 
two or more.  Mailman could be hacked to pull all addresses out of the
headers, then remove duplicates from the outgoing mail list.  Processing
time will increase significantly for very large lists, of course.

Chris