How can there be 2 kidneys in the human body?

Answer 1

People who lose a kidney due to trauma, disease or donation appear to do just fine so it seems that you truly have two in order to give you a spare. The kidneys are easily injured in traumatically (falls) and due to illnesses such as stones and infections and their function is vital to life so having two on opposite sides of the body provides a safety net.
The general design of the human body is one of bilateral symmetry; the body has a left side and a right side which are largely similar. There is only one brain, but the brain does have two hemispheres. There are two lungs, two arms, two legs, two eyes and two ears, and two kidneys. Having two of a given body part creates a backup system. So, you can lose a kidney and still survive.

Answer 2

The body is a mix of symmetrical organs, with mirror like symmetry from Right to Left sides, and non symmetrical systems.

The digestive system for instance is non symmetrical.

The kidneys however are part of the genito-urinary system, which does exhibit pairing of parts, and symmetry. We all typically have two kidneys, two ovaries, two testicles.
The exception to this 'rule' is when components of the system fall along the mid-line of the body, in which case only one part will exist.

Answer 3

Otherwise, onewould have to work too hard.