system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do Humans have multiple of some organs but not back ups of some very vital ones like heart, liver, stomach or brain?
gpt
The short answer is that every organ you have requires nourishment to be effective, and redundancies for everything would practically double the amount of food you would need to be healthy. While you do have two of some organs such as kidneys, they don't necessarily exist solely as backups. People with one kidney can't clean their blood as effectively as people with two kidneys, though they can still do it. Same with lungs, having both is definitely advantageous. 

Evolution isn't a designer with blueprints. Not having multiple organs isn't really all that harmful. Most people live plenty long enough to bear multiple children, even with their puny single heart. As such, because our current body structure is fairly effective, there was no evolutionary cause for change and natural selection doesn't necessarily favor someone with two hearts.