This extension of the original WordPress code allows you to post to multiple blogs by email. Normally when you post by email, the post is deleted.
This plugin solves that. It skips the deletion step, thus all emails are stored on the server to be posted on your other blog. But to prevent the same blog from posting the same email twice, it checks first to see if the post exists before posting an email.
Here's a link to the file to Blog by email. It's named wp-mail.txt. Just download, rename, backup your default /wp-mail.php file, and overwrite it with this one.
You can create a cron job to automatically check for new emails to post. Simply create a cron with this command:
wget -N http://your-domain.com/wp-mail.php
I schedule mine to run every 10 minutes