How would you prove an "independent" invention was really developed independently of the patent? You can't prove that someone didn't read something.
Basically, you can't prove 1 person did not read it. But, you can be positive 100 people did not read it. Now clearly this depends on the number of people working on the problem. So, while 100 is excessive in most fields, somewhere around 3-10 is probably a good lower bound.