A customer who comes back to a form no longer starts from an empty page. By default their earlier answers are filled in, they change what is different, and the button reads Update. If they empty an answer and send the form, that answer is removed from their record in Shopify.
A new setting, Form completion behavior in the form's What happens next step, lets you choose something else for customers who already answered: show a message in place of the questions, hide the whole tile, or start empty every time. Every send now also leaves a "Sent" mark on the customer that you can use in segments. More in Forms.