Why if i do:

Student student = (Student) request.getAttribute("account");
Course course = getCourseService().load(id);
getUserService().save(student); // throw expcetion

and instead if I do (I am only inverting 2 operations):


Student student = (Student) request.getAttribute("account");
getUserService().save(student);
Course course = getCourseService().load(id);
...

everything is ok?

student has a course with the same id, and duplicate objects having the same id is not allowed.
Either do it in correct order or use saveOrUpdateCopy

thanks!
But student has a set of courses loaded lazyly, so it shouldn't know if a course is in its set.

I'm almost sure it's the problem but you don't give enough info to confirm it.