How to avoid people to submit the form more than once?

I just had a customer let me know that she did not see my custom message on my form. My custom message says to email us if there is an error and we will edit the form. Instead, the customer resubmitted using another email address (our unique question field). I really want to stop people from submitting more than 1 form. If they don't see my custom message, they will go ahead and do it again. I tested it and I could see both my custom message and the default one below it. I don't what device or browser the customer who only saw the default message was using. Can you help? Thanks!

I can see in your form http://www.jotformpro.com/form/61527901988972 you have set up the field "Buyer's Email" as unique field, I have submitted your form once and then tried to submit the form a second time with the same email address, I got this message: 

Please do note that using a form field as unique question it is not probably the best way to avoid someone to submit the form twice, they may simply use a different email address and submit the form with the exact same  info they submitted previously.

I would suggest you to set up a different limitation to your form and set up unique submissions instead of unique question, this way you will get only one reply from someone, will be possible to submit duplicated submissions only if the user uses a different computer, web browser and if he / she is connected from a totally different internet connection. 

Here is a guide that will help you with that: How-to-Set-Unique-Submissions-on-a-Form

Hope this helps.