Note if a lesson is already part of a course when adding from the edit course page.

[ideadude]

Is your feature request related to a problem? Please describe.
The dropdown of lessons shows ALL lessons, even if they are already part of another course or even the current course. This can cause confusion if lessons have similar names or give users the chance to "steal" a lesson from another course accidentally.

Describe the solution you'd like
One option would be to note the parent course in parenthesis after the lesson name.

Describe alternatives you've considered
Another option might be to have a toggle to include/exclude lessons that are already attached.

Or both.

[andrewlimaza]

Note, some user's may want to have a lesson or two between multiple courses - https://wordpress.org/support/topic/lessons-for-courses-in-pm-pro/