This would be a great feature, and prevent double opt-ins or disappointed subscribers. Would like to be able to redirect people who double subscribe to be sent to a page of my choosing for each form or list.
Just a guess. You may be able to handle that like this:
When you create or edit a list, click on the Settings tab. Then click on the Options bar and uncheck "Allow duplicate subscriptions."
Then click on the Custom Pages tab, click on the Subscribe bar and enter your page address in the box beside "Failure page."