Hi Lorenzo,
Having a form button for a registration process is not a good practice as it adds a new row to your data and make it duplicate.
Instead, filter the profile page by signed-in user using the email (as each new email login will always add a new row for that email as you have seen), then enable the edit option in the tab.
Therefore, a user can only see and edit their own profile page, setup the registration process in that edit button just like how you do it for the form.
Also add a text saying “Please complete your registration process using the pencil button on the top right”.