Add subscriber to a Mailchimp group / update form answers via web request

I've a two-step newsletter signup form on a static webpage, which starts with a simple text field on the landing page with the email text field and a button.

I make a JSONP request to the POST endpoint of my custom Mailchimp form to subscribe a user.

When the user hits signup, I already want to fire the request to make sure that's done right way, even if the user decides to not give me any further information, but it also open a popup to prompt for a few more details about their interests, which I would like to organize as groups in Mailchimp. If I just try to re-subscribe the email address, this fails with the following error:

[xxx@yyy] is already subscribed to list [zzz]

So this approach doesn't seem to be working. What else can I do to achieve this user experience, without having to delay the request and potentially missing signups.