How to prevent users from sending duplicate submissions?

What will happend if one person will register 2 times or more?

Please note that by default, there is not restriction on how many submission a user can send. Any user can submit any number of submission but if you want your users to submit form only once, it is possible to achieve your requirement by taking advantage of "Unique Question" feature. 

Do you have the option on certain fields that says "don't allow duplicates"?

Yes it is possible to achieve your requirement. You can use our "Unique Question" feature to prevent users from submitting duplicate submissions. Example: You can set one of your forms question (like cell number or email address) to be a unique question. This will not allow a user to use the same cell number or email address to submit your form. You may like to take a look at the following guide which should help you:  http://www.jotform.com/help/223-How-to-set-Form-Limits-Based-on-a-Unique-Question

Hope this helps.

Do get back to us if you have any questions.