> A quick fix on your end, assuming you're also running procmail (a good
> assumption on this mailing list),
>
> Add the following to the front of your .procmailrc file:
>
> :0 Wh: msgid.lock
> | formail -D 8192 msgid.cache
>
> This discards duplicate mails based upon the message ID header.
While this recipe is on the procmailex man page, a recipe to
save duplicates into a different folder is a lot safer (like
the one further down in procmailex(5)).
Consider the case when mail delivery is interrupted after
the duplicate recipe, but before mailbox delivery. On next delivery,
the message is silently discarded although it was never delivered.
