This action kills cancer cells. Another problem with gene duplication is that it doesn't account for all, or even most, of the complexity in many systems. For example, the complexities of oxygen transport involve many genes which are not structurally similar. This is obvious when one considers anemia, a breakdown in oxygen transport. There are many causes which involve molecules with little or no overlap in structure.
Evolution can create systems from genes that are already around for other purposes.
One example is a gene for a specialized function in insects, which is similar to a gene used for a different reason in mammalian systems. This just pushes the question of irreducible complexity backwards from one system to some other. The machine analogy doesn't work because the original machine was designed, as is the new machine. Instead of the Third World mechanic analogy, I have an alternative analogy. An intelligent designer of nature is like a US engineer using a similar connector wire in both a lawn mower and a motorcycle.