The Event management should recognize the duplicate contactids that are trying to be added to an event in the form of registering or checking-in. For example when a customer has already signed up for the event from a portal but calls again to the company and an D365 user creates a new event registration manually there should be enforced preventation of this and a window should popup and say "This contact has already registered for this event". The same thing is needed with Check-ins, you can check-in a contact a hundred times if you want to.