CelloSaaS doesn't allow duplicate email address within tenant as well as across tenants, the reason being CelloSaaS has a feature called "User sharing" which uses UserID and email address as the unique identifier to share users of one tenant with another tenants.
For example, consider abccorp.com is building a multi-tenant education platform where the end customers are major universities, study centers colleges etc. A student (user) who enrolls for a course in one education center might be part of another university at the same time and both of the institution might be using the same application from abccorp.com. Here, creating a separate user/student from two different study centers leads to confusion, complexity, duplication and lots more. Instead, if both the universities collaborate and share the users with each other, they can allow seamless user management.
To learn more about Sharing User Feature, click here