Using emails

I'm very new to this Constant contact and trying to develop using PHP
Following is My Question:

When I tried to add information to List through PHP code.
If the email is already used for any of the contacts in the list, It is not allowing me to use the same email for another contact.

I had created a custom filed named CustNo. Can I use this field for
references. Can I use this filed as Primary than the email filed.
Usally we want to use this field for references insted of emails
So that I can add same emails for different contacts.
 
 
Aravind

re: More than one contact with the same email address

Hello Aravind,
Each contact in your account must have a unique email address.  This restriction is true in the online application and is enforced in the API. 
Thanks,
 

Tom Mignosa, Product Manager, Constant Contact