Overview:
Two users can have the same email but different logins! As long as the passwords are different, you can have separate users. One thing to be aware of is that if the "Forgot Password" tab is selected, it will ask for an email address. This then becomes tough to tell which user it is associated with. The best recommendation for this scenario should you forget the password, is to change it from the "Edit User" screen in Breeze.
Here are a few additional resources: