In Moodle 4.5.2, 4.5.3 (tested in our production and test instances, as well as on school.moodledemo.net), when creating a new course structure with sections containing empty subsections, duplicating such a section throws a DML exception error 'error writing to the database'.

The expected behaviour is that a teacher / course creator can create a 'template structure' of a course with sections and empty subsections, and duplicate these as often as required.

To reproduce:

1. Create a new course in sections format
2. Add two subsections to a section
3. Duplicate this section
4. Error occurs