In the past I was able to add multiple users using this format: "Lastname, Firstname" <email address>, but now it just doesn't work. What am I doing wrong?
I can't answer you as to how you were able to add multiple users with the same address in the past, as I have never been able to do that myself nor wanted to have multiple users with the same emails as it could be a violation of ferpa.
Below is a discussion that about reasons not to have student using the same email address that I think will help.
two students - same email address
No, not a single email address. Each student has their own email address. I was always able to add multiple users, each with their own address, at a single time. That is what I'm trying to figure out how to do...
Are you talking about adding students to a class on Canvas? Do the students already have Canvas accounts or are you trying to have Canvas create their accounts as part of the process of adding them to your course?
If you could post a screenshot of what happens now when trying to do what you are describing that might help us with an answer.
Retrieving data ...