it's actually not that uncommon. I spoke with my patent attorney some time ago and he could attest that very often the exact same inventions are sent to the patent office only days apart. Sometimes even on the same day. Famously Elisha Gray sent in his patent application for the telephone at the same time as Alexander Graham Bell. A letter was sent out to the two parties asking them to defend their application in order to find out who should be awarded the patent. Gray's company didn't find it worth the trouble to respond, so the patent went to Bell.
The point is that people and the ideas they have are influenced by external factors, many of them unconscious. But we will pick up on the same clues, think about the same problems, and maybe come up with the same solutions based on external factors. Nothing can stop an idea whise time has come.
There's also the Calculus and the primacy dispute between Leibnitz and Newton. The dispute itself isn't that interesting, but that something so complicated was created in the background of natural philosophy by two different people.
(I've recently read The Baroque Cycle again, does it show?)
The point is that people and the ideas they have are influenced by external factors, many of them unconscious. But we will pick up on the same clues, think about the same problems, and maybe come up with the same solutions based on external factors. Nothing can stop an idea whise time has come.