A student may use the same username and be in multiple classes.
A teacher uses the Search for existing students button to add the student to the additional class(es).
To Add Students to a Class
Choose CLASSES from the blue ribbon.
Choose a class from the left column.
Choose Roster from the middle column.
Click on the student symbol with the + sign on the right.
Choose SEARCH FOR STUDENT students.
Then search for the student(s) you wish to add to a class.
Use Column Visibility to display more student information.
Locate the student.
Click ADD.
The student(s) will show on your class roster.