Great question! At this time the basic sign up form is designed to update, or over-write information if its identical to other information in the account. This is actually considered an "update" to existing information. If you would like a more custom option it would be worth looking into a custom form. We've got a developer forum where you can do some research or here' s a link to our API page.
I'm happy to pass this feedback along for you. Can you tell me more about why the contacts would be signing up again? Are you asking them to make an update to their preferences?
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