GDragoN is right. While on the Wp-Admin dashboard, click on the GD Star Rating section (Appears after the settings section), click the Settings page (in the GD Star Rating section), click on the Articles tab, and look at the Restrict group of check boxes.
The checkboxes you will see are:
- Prevent article author to vote. This is only for registered users.
- Use logged data (IP) to prevent duplicate voting.
- Allow votes from user and visitor coming from same IP address.
Uncheck #2 and #3, and check #4 to accomplish your objective of allowing multiple votes.