Suppose someone comes up with a great original idea.
Some time later, someone else comes up with exactly the same idea without knowing that idea been conceived already and even without being influenced in any way by the other person's thought.
In other words, both of them make the discovery totally independently and not at the same time. For example, Leibniz and Newton invented Calculus this way; Cook and Levin established NP-Complete theory this way.
The idea from the very first inventor is no doubt considered original. But what about the same idea independently conceived by others later in time? I have no problem calling the latter original, although it seems less original than the very original one. Would there be a word or phrase that can readily convey the state of being "less" original?