How can I ensure a registration form doesnt have double entry.

You can use unique submission feature in order to avoid double submission. Setting the "Loose Check" or  "Strict Check" you may narrow down your target audience accordingly. Further information can be found in the following link:

https://www.jotform.com/help/64-How-to-Set-Unique-Submissions-on-a-Form

You can also use unique question feature. Setting one of your questions (like email address) as a unique question so that you will be able to prevent users who have same email addresses to submit your form. Further information can be found in the following link:

https://www.jotform.com/help/223-How-to-set-Form-Limits-Based-on-a-Unique-Question