Im having an issue with duplicated entries.

I tested on an iPhone. I sent in a submission then closed the safari browser. when i re oped the browser hours later it sent a duplicate submission. Is there any way to block duplicate submissions by default in a form?

This issue happens on mobile devices only as the page is reloaded and re-submits the data. You can set up unique submissions, so your users receive an alert when re-submitting the form. These guides will help you with this: 

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

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

Another option is to replace the Thank-You page with a URL, if you do not have a custom link, you may use this one: https://www.jotform.com/thankyou.html 

This guide will help you to change the Thank-You page URL: https://www.jotform.com/help/38-Redirecting-Users-to-a-Different-Page 

I hope this helps.