Hello Ben,
Thank you for reaching out to Constant Contact API Developer Support. My team is here to assist outside software developers with questions about building into Constant Contact's API.
While you can't use the same email address as the username for different Constant Contact account users, you can use the same email address as the email on file for that user. However, you'd need to go in and change the username for each user before creating the next user.
Example:
Username name1
Email name@domain.com
Username name2
Email name@domain.com
Alternately, a potentially easier way to do this might be to create the users with the "plus addressing" method, where the users are named like "name+1@domain.com" or name+account2@domain.com", but any password resets or communications are still sent to "name@domain.com" for all accounts.
[external link] What is Plus Email Addressing and How Do I Use It?
https://kb.uconn.edu/space/IKB/10731880518/What+is+Plus+Email+Addressing+and+How+Do+I+Use+It
Example:
Username name+1@domain.com
Email name+1@domain.com
Username name+2@domain.com
Email name+2@domain.com
Please have a look and let us know if you have any other questions!
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