"Add More" button to form
It's a well known problem of Drupal 7 - every form button is rendered with type="submit".

What you need to do, is to add prevent="click submit mousedown" to your button, to prevent actual submission. Maybe only one of those 3 is really needed, but this combination gave me pretty consistent results.

OK, but the above only prevents form from actual saving data. It still gets validated, and that's not exactly what you want, right? I already got answer on how to omit form's validation at AJAX button click.

Other approach would be to either add custom form element, let's call it real_button, a copy of button but with type="button", or to change type on document ready using jQuery.