public void AddStudentDuplicated_Test()
{
var enrollment = new CourseEnrollment(
courseId: "Course_1",
courseVersion: 0,
courseTitle: "Course 1",
capacity: 2,
students: new string[] { "Student_1" },
summary: new CourseSummary()
);
var student = TestHelpers.CreateStudent(id: 1, age: 30);
enrollment.Invoking(it => it.AddStudent(student))
.Should().Throw <DomainException>();
}