Tonight I reviewed the mail flow for bugs@darcs.net. Here's some details from that: 

- The mail is managed by exim4. 
- It appears that the bugs address is managed throug here:
  /etc/exim4/conf.d/router/459_bugs

Which has these contents:
darcs_bugs:
  driver = redirect
  local_parts = darcs-bugs
  senders = bugs@darcs.net : bugs-bounces@darcs.net
  user = www-data
  data = "|/usr/bin/procmail /etc/darcs_bugs_procmailrc"
  pipe_transport = address_pipe
#####

The procmailrc file it refers to looks like this:
###
:0 chfw
SUBJECT=| formail -xSubject: | perl -pe 's/\[darcs-[^\]]+\]//'

:0 hfw
| /usr/bin/formail -i "Subject: $SUBJECT"

:0
|/usr/bin/roundup-mailgw /var/lib/roundup/trackers/darcs

####

I found it notable that Roundup includes documentation for integrating with Exim
( search for Exim on this page:   
  http://roundup.sourceforge.net/doc-1.0/installation.html ).

However, the examples they cite there have noticeable differences from the
system we are using. 

So, without fully understanding how Roundup or Exim work, I advise evaluating
configuration changes that would bring us more inline with the official
Roundup/Exim example. 

I hope that might solve the dupe bug reports that get happen frequently now.