Message-ID as unique key?

Hi,
I am futzing around with Andrew Stuarts "Catchmail" program
that stores emails into a postgresql database.

I want to avoid inserting the same email more than once...
(pieces of the email actually get emplaced into several
  tables).

Is the "Message-ID"  header field a globally unique identifer?

I eventually want to have a cron job process my inbox and don't
want successive cron tasks to keep re-entering the same email :)

Jerry