The event registration UI ignores existing reservations, so I am able to register (and pay!!!) for the same event mutliple times. While this is something most users would avoid doing naturally, it's very likely to happen in my appliation, where users are registering for weekly bus rides.
Ideally, the registration and event details UI should show you that you've already registered and allow you to update/edit or cancel your reservation. This functionality is only available through the history table currently. It's really confusing for users who will see different info and options based on how they are viewing the event.
