Michael Simpson on "[Plugin: Contact Form DB] Preventing duplicate e-mail addresses in database"

I haven't tried this, but you might try this:
- create a file cfdb-filters.php at the top of your WordPress installation
- On the first line put:
<?php
- after that put in your code
- In wp-config.php at the very end of the file add the line:
require_once(ABSPATH . 'cfdb-filters.php');

Let me know if it works.