Gene duplication and subfunctionalization
Suppose a gene performs two distinct functions 1 and 2, both of which are essential. We'll call this gene A, for ancestral gene.

A duplication copies A to another locus. Now there are two genes, A and B, that perform the same two functions. At this point, if either A or B is deleted, the organism is fine.

Over evolutionary time, A and B undergo genetic drift to become A' and B', respectively. Let's say that B' loses function 1, and that A' loses function 2. This isn't a problem for the organism since A' still performs function 1, and B' still performs function 2. In other words, the two genes complement each other.

However, now A' can't be deleted without losing function 1, and, similarly, B' can't be deleted without losing function 2. The result is that both genes are fixed in the genome by evolution.

This means that now, after hundreds of millions of years of relevant evolution and genetic drift, we many duplications like A' and B'. The key to this is that through genetic drift with no selective pressure, it's very easy to lose function but extremely difficult to gain it.

As for your second question, I'll look for a paper on gene duplications that I think would be the most helpful, but there are many.