Binary planetary system
A binary planetary system, also known as twin, double or sister planets, were planets whose features were more or less identical, or at the very least similar, and were usually found nearby each other. This included the lifeforms which inhabited them.
Known twin planet systems
Earth and Mondas were so similar that even the shapes of their landmasses were identical, albeit vertically flipped. Mondas had lifeforms identical to humans called Mondasians (TV: The Tenth Planet) and lifeforms identical to Silurians called Lizard Kings. (COMIC: The Dead Heart) New Earth also had the same orbit, size, atmosphere, and terrain as Earth. (TV: New Earth)
Other examples include Atrios and Zeos, (TV: The Armageddon Factor) Androzani Minor and Androzani Major, (TV: The Caves of Androzani) Thoros-Alpha and Thoros-Beta, (TV: Mindwarp) Raxacoricofallapatorius and Clom, (TV: Love & Monsters) Gemino and Gemina, (COMIC: The Secret of Gemino) a system in the Tornado Nebula, (TV: Sky) Antari Two and Antari Three, (PROSE: First Frontier) Caludaar and Endarra, (AUDIO: Scaredy Cat) and Dinasis and Bagoss. (PROSE: The Mondas Touch)
