Is it possible to disable the option that allows users to sign up twice? Currently, it's possible for someone to book multiple appointments on different dates using the same name and email address. This leads to situations where people forget about one of their appointments. I would prefer that users can only book one appointment at a time. If they wish to book a new date, they should first cancel their existing appointment. Ideally, a pop-up or notification could inform them about this. Would this be possible?