Binary planetary system

A binary planetary system, also known as twin 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) 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) and Caludaar and Endarra (AUDIO: Scaredy Cat)

There were also other planets which were very similar. New Earth was a planet which had the same orbit, size, atmosphere and terrain as Earth. (TV: New Earth)