If you've submitted the form and you press f5 or ctrl+f5 it will post the form back and resubmit your data. This is not a deficiency of Kentico, it would do this for any asp.net application you would use or create.
If you simply want your users to continue to re-enter data then in your form setting redirect them to the current page with the form on it. You could also pass a URL parameter like
?resubmit=true and place another editable text webpart on the page template and set the visibility based on the querystring value. This way you can display a "Thank you for submitting your data. Submit another." type message.