We have several lists for different interests. We also have an opt-in form where the user can indicate their interests by checking boxes for any lists they’d like.
I’ve noticed that if a user completes a couple checkboxes during one visit, and later completes the form again and chooses different checkboxes, they are not added to the new lists (the checkboxes selected during the second session that were not selected during the first session).
How can I make the form update their list selections in an additive way? In other words, if they don’t select a checkbox the second time that they did the first time, I don’t want them removed from that list.