How to prevent duplicate submissions from user?

Our form has the first name, last name, email, phone, zip info and we are seeing few entries with the same zip but with different name, email and phone - but they all look made up and look very similar in structure - but surprisingly, all of them are coming from different ip@.

Also, since the emails are invalid ones, they bounce when we try to send a test message.

We are pretty sure these are bogus ones and trying to find a way to filter them out - any suggestions will be much appreciated. 

Thanks.

Have you received SPAM submissions? In which form you have received those SPAM submissions?

The first step I would suggest you to please add a captcha field in form and enable reCaptcha option. This will ensure that you do not receive SPAM submissions using a bot. Please check the screenshot below on how to enable reCaptcha:

Please note that it is also possible to prevent multiple submissions from users and here are the options available:

#1. Unique Submission: You can take advantage of our unique submission feature to block multiple submissions from one system of one IP address by setting the "Check Cookies Only (Loose check)" or "Check Cookies & IP address (Strict Check)" respectively. Here is a guide to help you set unique submission:  http://www.jotform.com/help/64-How-to-Set-Unique-Submissions-on-a-Form 

#2. Unique Question: You can also use our unique question feature to prevent 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. Here is a guide to 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.