关于EJB3.0的多对多映射问题

[bet_fov]

教师和学生是多对多关联，为此我做了三张表：教师表，学生表和中间表。在对这三张表做测试的时候，我写了这么段代码：
//教师为关系的维护端
TeacherFacadeRemote sdao = (TeacherFacadeRemote)ctx.lookup("TeacherFacade/remote");

Teacher teacher1 = new Teacher();
teacher1.setTcourse("徐德志");
teacher1.setTname("数据结构");

Teacher teacher2 = new Teacher();
teacher2.setTcourse("罗三定");
teacher2.setTname("数字图像");

Student student1 = new Student();
student1.setSclass("软件0302");
student1.setSname("王博");

Student student2 = new Student();
student2.setSclass("软件0301");
student2.setSname("阿标");

Student student3 = new Student();
student3.setSclass("软件0302");
student3.setSname("全中");

Student student4 = new Student();
student4.setSclass("软件0301");
student4.setSname("曹梅");

teacher1.getStudents().add(student1); //teacher1带的学生是student1
teacher1.getStudents().add(student3);

teacher2.getStudents().add(student1); //teacher2带的学生也是student1
teacher2.getStudents().add(student4);
teacher2.getStudents().add(student2);

sdao.save(teacher1);
sdao.save(teacher2);

这样学生表会出现两条student1的纪录，但我只想让学生表存在唯一的学生纪录而中间表里可以存在一个教师对应多个学生这样的纪录，请问有什么办法？
附：
Teacher.java
@ManyToMany(cascade = { CascadeType.ALL }, fetch = FetchType.LAZY)
@JoinTable(name = "teacher_student", catalog = "ejb", joinColumns = { @JoinColumn(name = "teacherid", unique = false, nullable = true, insertable = true, updatable = true) }, inverseJoinColumns = { @JoinColumn(name = "studentid", unique = false, nullable = true, insertable = true, updatable = true) })
public Set<Student> getStudents() {
return this.students;
}
public void setStudents(Set<Student> students) {
this.students = students;
}

Student.java
@ManyToMany(cascade = { CascadeType.ALL }, fetch = FetchType.LAZY, mappedBy = "students")
public Set<Teacher> getTeachers() {
return this.teachers;
}
public void setTeachers(Set<Teacher> teachers) {
this.teachers = teachers;
}