The Contact information on the Event Page is for the Event Booker and it is mandatory to process an Event Booking.
If you set Contact matching module on, under Module Settings, the person making the booking can click on the Attendee information button to populate their information.
This will activate the following button. When it is pressed it will display the attendees that were entered.
You could also add a Narrative to the Contact objects explaining what the user should do if they have already entered their details.