A theory like relativity was probably discovered for the first time by Einstein, what with the essential preceding discoveries (that Einstein had read) and time-tracking devices available. So that he could reason about the ticking of clock hands as an observer in a fast-moving vehicle.
But I wonder about novels and how many plots or characters are extremely similar if not said to be almost identical. Or even ecology, how much does a modern ecologist discover now that has been known by the natives for centuries?
How many great ideas have we forgotten? Because culture doesn’t always move in the right and true direction on its own. And now we try to repeat them, starting from scratch. Instead of standing on the shoulders of giants.
Any one person in one lifetime builds mental models and avoids redundancies in their memory. Over time from generation to generation, however, this isn’t always the case.
So it becomes important to take your knowledge not just from your space and time but look back in history and across cultures to find completely new (to you) schools of thought that can fill important gaps.