As usual, the setting created by a conspiracy wingnut runs on Star Trek logic. Also, I thought in Star Trek, they tried to explain it away in one episode, by an ancient race of space humans seeding planets a long time ago.
If you start to think about all these television tropes, barely any of them make sense. Unless panspermia happened, I would doubt that any alien life would have DNA. And even if, that life might have different bases, or interpret codons differently. That assuming it is even carbon-based life. This might be a fair assumption, but carbon can do much more than what we considered “organic”, see our plastic and polymers.
And then you get to the environment: no way would an alien planet arrive at the exact same conditions as earth. You could have all sorts of configurations of atmospheric composition, pressure, temperature. Even if you assume liquid water, instead of methane or something: we are at the lower range of it being liquid, I think high-temperature life around 50°C or so would still be plausible.
Life existed for a while on Earth before the Oxygen Catastrophe happened, so we know oxygen is not only not obligatory, but also harmful to life not adapted do deal with it. It also implies that if life came to Earth via panspermia, that life was anaerobic. And even if it contains oxygen, would it have it in the concentration we are used to?
(The temperature and oxygen thing could also mean something interesting: if the chemistry you are based on is significantly slower or faster than ours, you might be leaving on a wholly different timescale)
As for mining, there is absolutely no good reason to actually mine a planet, if you have spaceships. Any mineral should also be available on asteroids, or small moons/dwarf planets, where you don’t have to lift it out of a deep gravity well. Even less so an inhabited one, where the native life might interfere.