The Doctor may not realise it at first, but he's been to Planet 6767 before…

The new 'Doctor Who' episode 'The Well' is actually a sequel to a David Tennant classic

Source: space
Source Link: https://www.space.com/entertainment/space-movies-shows/the-new-doctor-who-episode-the-well-is-actually-a-sequel-to-a-david-tennant-classic