This depends on what your script does and how your site works.
If you require a password or other unique name to vote, then the script must capture not only the vote, but the unique name as well. Then, as votes are cast, the script should check the "votes database" for the unique name BEFORE adding the vote to the database.
The same holds true if you don't require a unique name but are capturing the voters name at time of voting. If the script doesn't check the "database" for existing names then you run the risk of multiple votes.
If you're doing any of the above there's really no sure method of stopping the multiple voting process.
There's always a better way. The fun is trying to find it!