This quote is from this thread: http://developer.constantcontact.com/node/1264
"The reason this is not working for you is that the functionality of the Constant Contact Signup Form Generator does not allow contacts that are already signed up to the account, sign up again.
If a user is already in the account, even if they are on other lists, or even if they are on the Removed list or the Do not mail list, they are still considered a contact."
I would like to cast my vote for fixing this. Especially the part that even if they were removed from the list, they cannot use a CCSFG form to sign up again.
Thanks very much for your feedback. Regarding the functionality to update contacts, our main concern is the security of the information contained in the accounts and we want to make sure that all information added through the CCSFG is accurate. With that being said, this is something that we would really like to implement for obvious reasons. While I can't confirm for sure that we are going to be adding this functionality, it is something we're looking at very closely.