I have a custom post type 'Course' and custom field group 'Materials'; 'Materials' is the repeatable group to the custom posts 'course'. How can I duplicate the custom post course with all custom field group content together?
As far as I can see if I'm not wrong the plugin "Duplicate page" does not offer any hook.
Internally, the repeating field group is managed as one-to-many post relationship. So, all entries you add to repeating field group will be added as child post and parent post will be to which you attached and set your repeating field group to display.
For instance, if you have post type "Students" and you added repeating field group namely "Skills" having fields (skill name, skill description, skill level) and set this repeating field group to display on "Student" post type. Then there will be one-to-many post relationship between post type "student" and "skills" (repeating field group slug will be treated as post type automatically).