There isn't a conclusive answer, but basically it appears that humans have two kidneys just in case something goes wrong.

If one kidney is damaged or removed, the other can grow and change enough for survival. One might expect that the extra work done by the remaining kidney would speed failure, but so far that does not seem to be the case. (However, it is possible that people die from other problems before the effects can be detected.)

You're actually healthier if you do have two kidneys. We don't *need* two, but it benefits us to have two, so we do.

We have 2 lungs, too. We can donate 1 of our kidneys or 1 of our lungs.

We have 2 sides of the brain, and 2 eyes, ears, nostrils, but only 1 heart. Our 2 eyes and 2 ears help us see in 3D and hear where a sound is coming from. Our 2 arms and 2 legs help us move and do things.

We and lots of other animals have this symmetry, where there are many things the same on our 2 sides. It's how we evolved. Here's an article in wikipedia about our 'bilateral symmetry' and all the other kinds of symmetry in living things:
symmetry in biology