What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
I would like to start of by giving my suggestion. It would be to register for free account while you make your tests. This will ensure that your jotforms are not lost and any changes that you make that are saved for you. As it currently stands I can see that you already have 2 guest accounts each with 1 jotform and registering you would be able to access both from the same account no matter if you change the device or not.
may we implement a double check (user must input 2 times his email)
Yes, you can do that using Field confirmation widget.
You could also use this widget E-mail Validator - Verification widgets.
Both widgets are great at what they do and you can use either of them or both on your jotform.
You can see here How to Add a Widget to your Form.
If you have any further questions feel free to let us know.