That's pretty ridiculous. The purpose of the law is to keep people from unfairly copying ideas, not rediscovering them.
However this is different. It's not a person rediscovering it, but an algorithm which happens to do exactly the same thing but is provably not unique or patented. It's a different algorithm that just happens to converge on the same process.
This is perfectly analogous to patents on real world inventions - there are plenty of cases of different machines that do the same thing. You can't patent the output of an algorithm, just the process by which it gets to that output.
You can be liable for triple damages if the court decides you knew about the patent and infringed anyway. Normal damages if you did not. I believe independent invention is meant to be so rare as to be suspicious (despite many documented cases where this happened).
The (supposed) purpose is to give an incentive to inventors to freely broadcasting their inventions, by giving them a monopoly on it so they can recoup their investment. Without that incentive, inventors would keep their inventions trade secrets.
