#3 Reject duplicate incoming messages

[Geoff Schmidt]

Some lists will desire to publish a Mixmaster nym or
reply block,
instead of their actual address, in order to conceal
the physical
location of the list server. Unfortunately, the
Mixmaster network
is not guaranteed to not drop messages on the floor.
Typically,
one addresses this problem by composing reply blocks that
send several copies of a message by alternate routes,
increasing
the likelihood that at least one copy will be
successfully delivered.

To facilitate this, cryptlist should optionally hash
the body of
each message and store these hashes temporarily in a
database
for the purpose of rejecting messages that have been seen
before, so that the list does not receive duplicate
copies of
a message when the remailer network functions properly. The
hashes should expire from the database after a configurable
length of time, which should default to several days to
a week.