You cannot have multiple contacts in Event Temple using the same email address, as the email address is the unique identifier used by the system. Otherwise, the system would not know which contact you are referring to when you ask it to do something with that address. 

Potential workarounds include;

slightly modify the email by adding an additional number and/or symbol eg support+1@eventtemple.com in order to add the person into the system. Keep in mind is that when it comes to emailing them, you'll have to ensure you choose the correct contact. (eg the contact "Support team" who has the email support@eventtemple.com).

Another option is to use the generic contact 'Support team' for the Booking, and then note the person's name you are dealing with within the Booking (perhaps using a custom field). There are then considerations to take into such as merge fields in documents & email templates.

Steps on editing a contact can be found here.