Within a course
Click the three-dot menu (⋮) next to the lesson or chapter and select Duplicate. The copy is created in the same course.
To another course
Chapters can be duplicated directly to another course: three-dot menu → Duplicate → To another course → select target course.
For individual lessons, there is no direct option. The workaround:
Create a new chapter in the source course
Move the lesson into it
Duplicate the chapter to the target course
Delete the temporary chapter in the source course