We've recently taken on mailman to handle many large, popular lists at my day job, and one strangely-missing feature is the inability to avoid duplicates when someone cc's a list's old alias and new alias (we also moved the lists to a subdomain, out from under our primary).
It's suggested in the FAQs that this could be done using procmail -- but I'd think it (could/should) be done in mailman itself.
The list-id header is ideal for this, and it SEEMS to be not more than a dozen lines of code to do the same sort of per-list "lockfile" that the procmail recipe does (plus add a variable to make it tunable in either the config file, or the web-ui). Presumably lockfiles could expire after 7 days. Locks could also be an entry in a .pck database.
The question is (and the reason I ask this on a developer list rather than -users) -- is there anyone here who could provide a patch to add this functionality, or who is interested in taking on such a task for some sort of compensation/donation?
Please contact me off (or on) list if anyone's available.
Another great feature would be to have mailman "strip" multiple cc recipients. I.e. if a message is sent:
to: list cc: list-alias(*), another-list-alias(*)
to: person cc: list, list-alias(*)
To have these (*) stripped (and prevent the need for this). But that's more work, and right now the duplicates are a major regression from what we had before.