MDL-56972 backup: fix question category stamp duplicates during restore
MDL-56972 backup: fix question category stamp duplicates during restore

To kerb future stamp duplication, MDL-54864 added a unique index to the
question_categories table (stamp, contextid) and added an upgrade step
to fix existing duplicate stamps. It missed the case where a course with
duplicate stamps in the same context is directly restored into moodle
3.1, causing index clashes. This issue provides a fix for that.

dilawar/moodle