Why is EF7/Core inserting duplicate records?

In my domain model I have

class School
{
    public ICollection<Student> Students { get; set; }
}

class Student
{
    public ICollection<Course> Courses { get; set; }
}

class Teacher
{
    public ICollection<Course> Courses { get; set; }
}

class Course
{
    public Student Student { get; set; }

    public Teacher Teacher { get; set; }
}

Say I already have a Student and a Teacher in my database

Student ID = {C26A6D90-6D36-4CE9-9FF0-5C5BAB6994CC}

Teacher ID = {26ECCCC6-4A24-4FB2-A406-5021B693EE5E}

And then I want to create a new course

Course c = new Course(student, teacher);
repo.Add(c);
repo.Save();

1. I get an error telling me I can't re-create a school with the same ID
2. I get an error telling me I can't re-create a student with the same ID

basically, I'm getting the records from the database (Student & Teacher), and then trying to use them to create a new relational entity (Course)

{System.Data.SqlClient.SqlException: Violation of PRIMARY KEY constraint 'PK_Schools'. Cannot insert duplicate key in object 'dbo.Schools'. The duplicate key value is (78434ef7-20db-42c8-448e-08d401d04461). The statement has been terminated.

This seems to be an EF7 problem...is it just not mature enough yet - because it doesn't feel like it!