Check to see if the email address you're using for that new contact is already in Aero. (Manage > Email Addresses) Email addresses can only exist in Aero once.
If the email address does already exist, here is how you can add a new contact that uses that same email address:
- Add the new contact with no email address
- Edit the contact you just added and select the correct email address from the email address contact
- If that new contact is the only one you wan tot be associated with that email address, edit the email address and select the new contact as the default for that email address
- Then edit the other pre-existing contact and remove that email address as the default (if necessary)