I had same problem when I click grad link and execute backup.
After I trace code and query database(use below sql) than I got duplicate record.
SELECT * FROM mdl_grade_items where courseid= xxxx and itemtype= "course"
SELECT * FROM mdl_grade_categories where courseid= xxxx;
I have no idea where did they come from ? So I just delete it, and it's work for me.
My Moodle versin is Moodle 1.9.9 (Build: 20100609)
I Hope that this is going to help you.