Auto-duplicate mailstore?

I am looking for something that is similar to replication, but without a second server. I currently have a system setup using postfix bcc to write out copies of all email, but what I would rather do is just have all the mail written to /usr/local/virtual/%u/Maildir and also to /backup/usr/local/virtual/%u/Maildir (for example) as this will make restoring messages much simpler.

-- Gods don't like people not doing much work. People who aren't busy all the time might start to think.