I created a sign up form on my website that has a lengthy questionnaire and this is for people to subscribe to my subscription service. I am having issues because if someone is already signed up through my join my mailing list form on constant contact it won't let them fill out this form and get added to these other lists and ultimately subscribe to my service.
You would need to modify the code behind your signup form to query for the email address being submitted in your account, and then add the contact if it is new, or update them if they are already in the account.
We have example code here for our newest PHP library, in the examples folder called addOrUpdateContact.php; if you're using a different library, we may have example code available. Just let us know which library you're using (this page shows a list of most of our wrapper libraries).
API Support Specialist