Actually we only have one of most organs. Duplication of organs is the exception, not the rule.
Evolutionarily, it does not make sense to duplicate organs. In nature, injuries serious enough to prevent any one organ from functioning are almost always fatal. It's not like lions eat one kidney and then run away. Biologically duplicating an organ is really a massive waste of energy, why do we need two kidney's is a better question? The answer is sometimes there's an evolutionary advantage to have split-process systems. With kidney's, maybe smaller organs instead of one larger one is more efficient. Or possibly kidney's have a high rate of blockage and that means swift death to septic shock. By having two, one can keeps functioning while the second repairs itself. Liver's on the other hand are not prone to blockage in the slim, healthy people homo sapians were for 6,000 of the last 6,003 generations.
So the answer is that we don't have two of most things because having two is not an advantage. We're not the Borg, we can't have backups and spare parts. We're a single unified system that sometimes has a little bit of duplication, but only rarely. Skeletal radial symmetry in vertebrates gives an illusion that we have two of many things, but our outward physical form is not what keeps us alive.