First Law of Planetary Building
First Law of Planetary Building: no two planets will ever be alike.
Corollary #1: if two planets are almost identical, then at least one of them will have at least one outrageously peculiar feature.
Corollary #2: Universes made of perfectly identical planets are not allowed.
The First Law is manifest in the fact that each planet in the Solar System and elsewhere appears to be a unique, very specific experiment with peculiar conditions that are never repeated elsewhere. Even single satellites are all very different from one another. And if you want to top strangeness, how about Corot-7b with its clouds of minerals?
One objection could be raised about Venus and Earth, or Uranus and Neptune, as both couples look like made of identical twins. However, Venus’s hellish atmosphere and very slow, retrograde rotation are truly outrageously peculiar features; and Uranus basically lies to one side (hence corollary #1).
Corollary #2 is necessary otherwise the First Law is invalidated. It seems plausible, since the number of universes is large but not infinite.