We have two kidneys not so that one of them can be a spare, but because they evolved from structures that were already pairs. We never gained an “extra” kidney… we still have two kidneys. In most organisms with dedicated kidney-like-organs, you will find these in pairs along the body, probably due to bilateral symmetry during embryogenesis.

Why don’t we have two of other things, for redundancy? Well, unfortunately, the best response to evolution questions is often: ”why would we?” The benefit of evolving an extra of a given organ would have to outweigh the cost. That’s putting energy and time into something that the organism ideally will never need.

Two kidneys for a human-sized organism is pretty cheap, evolutionarily speaking, for your osmotic-filtering-needs. Annelid worms have two nephridia per segment. But two stomachs, two hearts, four lungs? The “cost” rises very quickly for these structures.

That cost is very high when the only time you would need a spare is a life-threatening injury. The only benefit it confers is a higher chance of survival upon taking catastrophic damage. Organisms that sustain life-threatening injuries don’t often survive them - if you’ve taken enough damage to irreparably damage an organ, the chances of you surviving that are not particularly high, even if you have a spare.