It's a big world and there are a lot of smart people in it. History is full of examples of multiple people independently inventing/discovering the same thing.
May I also observe it isn't me making a normative claim that such things that are easily re-constructed by anyone should not be patented. It is descriptive, right in the legal definition of patent; it should be not obvious to one skilled in the art. Again, the very fact such a clause is necessary is evidence that there is a set of things obvious to one skilled in the art that exists, or why bring it up?
So we need something to get around the problem of 'see new thing - figure it out - declare it obvious - sue to overturn patent'. Maybe the proposed first-to-file requirement will help that by taking the patent office out of the history-investigation business and just acting as a recording service, letting the parties fight out their dispute in court; I hope so. But it's hard to come up with some catch-all definition of what constitutes 'obviousness' because that will always be open to both honest and mercenary dispute.