Approval of duplicate message ids

Last night, somebody seems to approved a bucketload of duplicate
messages to pgsql-general, which judging from their headers were all
injected by the same mail-agent fart at deltacom.com a couple of days
ago.  I know that the mail list software recognized these as duplicates,
because back when it happened I got bounces related to the ones that had
originally been from me.  But apparently, nonetheless they went into the
moderation queue and somebody whose sense of deja vu wasn't strong
enough eventually approved them.

Why is that?  Why aren't duplicate messageids/checksums rejected out of
hand?  Or at least clearly flagged to the moderators as something they
should almost certainly reject not approve?

[ looks a bit closer... ]  Also, it might be a good idea to summarily
block mail.deltacom.com until they fix their problem, because it looks
like there was a second batch of dups submitted yesterday.
        regards, tom lane