View the answer-comparator source.
Answer Comparator compares the current form entries to those that have already been submitted.
You can see the script in action here.
Steps to use the script:
Download the source for your form.
Add the following lines of code just before the closing BODY tag:
Include these files in the directory with your downloaded form. md5.js is used for hashing emails so they can not be scraped on the front end. It can be downloaded here. If you are sure that your form will not contain emails, this functionality can be removed (from answerComparator.php).
Include the JotForm API PHP file in your directory.
Include your API key and FORM ID in answerComparator.php.
You are all set!