A person can be added to a list multiple times, but the list will only contain that person listed one time.
For the smart list subscription, did you verify the subscription was active? You can check this in the topmost "Marketing Activities" folder under the "Subscriptions" tab. Also verify that if you have a list of recipients for the subscription to separate out their email addresses with a colon or semicolon (ex. "email@example.com;firstname.lastname@example.org")
I see. I was thinking I could have a trigger to add them to something every time that person fills out the form. The only thing I am going off right now is the Lead Automation Trigger Date, which I have been told is overwritten every time someone fills out a form. Do you know if this capability will be something that is available if Marketo is synced with SalesForce and can pull a report from SalesForce?
They are not in the Marketing Activities but are in top most Database subscription tab. I have no end date set and it says the campaign is active.