From this NYT article: “It’s rare that you’ve got a major breakthrough that wasn’t developed by multiple people at about the same time,” said Mark Lemley, professor of intellectual property at Stanford Law School.
On both an ethical and economic level, liability for independent invention is a problem. Imagine that you have put great effort into developing a new technology, start marketing it, and then get a letter in the mail telling you that another party has already obtained a patent on your work, and you now owe them money for putting out the product you’ve developed. There you were, minding your own business, not copying anybody, and now you suddenly owe a stranger perhaps millions of dollars. Ethically, it is hard to justify the justice of the situation.