Contact cannot re-register for an event

If a contact record has an event registration with a canceled status, they are still not able to have another record for that event. Instead an error is thrown: "This contact has already been assigned to this event."

Users should be able to cancel a registration and re-register someone at a later date.
They could update the status and change it from cancel to registered but any current payments won't be associated with this status change and there will be no obvious record of the person having canceled and then re-registered.

Overall, I see no reason why the participant record should be unique when the status is different.