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Also, how much were these men a product of their times and the stage in scientific thought at the moment, and how much were they 'ahead of their time'?

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Multiple independent discovery is extremely common in science and mathematics. My favorite example is the HOMFLY polynomial, which is a knot invariant that was discovered by no less than 5 separate groups of mathematicians independently at the same time ("HOMFLY" is the initials of most of them).