Non-unique Message ID in mail messages

[Marc]

> I apologize for bringing perhaps trivial/well-known matter, but I am
> interested in your opinion.
> 
> RFC 5322 clearly states that mail messages SHOULD contain a Message ID
> identifier, but if the do contain it, it MUST be globally unique.
> 
> Despite this requirement, I have encountered senders (namely Spamcop)
> that sends obviously different (albeit related) messages called "Alert"
> and "Summary" (they are always related to the same incident and have the
> same Message ID). This creates all sorts of problems when processing
> these mails, namely with users that have local forwards from one domain
> to another (our mailserver hosts multiple domains), because for example
> Dovecot refuses to forward the second message, flagging it as a duplicate.
> 
> My question to you is - did you also encounter similar incorrect
> (according to RFC standards) problem, and if so, is there a way to
> persuade dovecot to perhaps determine the uniqueness of a message by
> other means than just checking the message ID (i.e. look at other
> identifiers, Subject, perhaps)? Because according to the log records,
> Spamcop does not seem to be the only offender.
> 

I would think this is more related to MTA's then dovecot. It is dovecot's core job to put messages in mailboxes. 
However interesting this globally unique. At first sight, I would say a bit unnecessarily broad.
I would recommend using a mail filter in any mta, grab whatever you want and analyze that, with that solution you can add any header you like. 
I do not get what spamcop has to do with this, afaik this is a dnsbl.