Prevent duplicate participants when using offline event registration

There seems to be no benefit to allowing a contact to be assigned to an event more than one time by a database administrator. And, having duplicates can cause a major headache when you are trying to do outreach (e.g. phone banking for turn out).

And... adding duplicate records is very easy when you use the "Add to event" Action after doing a search. It's quite common to add many groups to a single event when doing outreach. Since the same people may be in each group, it results in many duplicates.

It would be ideal if, when adding a single person to an event, you get a form validation rule preventing a duplicate. And, when you are adding multiple people to an event, the duplicates are not re-added, and you get a single status message letting you know how many people were not added because they have already been added.

This should only apply to offline registrations.