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.