When Gene Roddenberry brought his "Wagon Train to the Stars" back for a sequel television series, there was no guarantee it would work. Fans and critics thought Star Trek was a fluke phenomenon that couldn't be repeated. Obviously, they were ultimately proven wrong. Yet, early in the run of The Next Generation, the producers made a huge mistake with the ship's doctor. The USS Enterprise-D had two chief medical officers, and the show should've kept both characters.
Source：https://www.cbr.com/star-trek-tng-ship-doctor-problem-solution/
