Multiple independent discovery or invention.

Wikipedia discusses multiple discovery here.

Historians and sociologists have remarked on the occurrence, in science, of "multiple independent discovery". Robert K. Merton defined such "multiples" as instances in which similar discoveries are made by scientists working independently of each other.^[2] "Sometimes the discoveries are simultaneous or almost so; sometimes a scientist will make a new discovery which, unknown to him, somebody else has made years before."^[3]

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From the 80's - a remarkable scanning job.

A few studies suggest that anticipated research is an occupational hazard for the majority of active researchers. Indeed, some investigators report being anticipated several times in their careers. 

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One of the most commonly cited examples is the independent formulation of the calculus by Newton and Leibniz, which has been definitively described by A. Rupert Halls Another is the theory of the evolution of the species, independently advanced by Charles Darwin and Alfred Russel Wallace.
But multiple independent discovery is not limited to only a few historical instances involving the giants of scientific research. On the contrary, Merton believes that multiple discoveries, rather than unique ones, represent the common pattern in science.

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From the International Journal of Pure and Applied Mathematics - a PDF file that I am tentatively offering a link to.  You may need to search "A MODEL FOR CO-DISCOVERY IN SCIENCE BASED ON THE SYNCHRONIZATION OF GAUSS MAPS" to find it yourself):

It is suggested that the driving external excitation that causes synchronization of systems and therefore co-discovery in science is the information access and its broad distribution in modern society produced by radio, tv, internet, books, newspapers, scientific articles among other media. It is concluded that co-discovery in science is a phenomenon which will be every time more frequent. For our 21th century we estimate that of the order of thousands co-discoveries will occur.

I should point out that the English is interesting - as an ESL instructor, I cannot complain about it but if you choose to follow the link and download the PDF, be warned that it is heavy slogging.  The math lexicon is also challenging.
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All this to introduce, not a scientific discovery, but a card game that appeared to be cloned from another but wasn't.

Could you design a brand-new game using only a deck of classic playing cards? It’s a cool idea -- repurposing familiar components in an original context. But the design for the game that would become Donsol was born out of necessity, the mother of invention. A pack of cards was all the creators had on hand.
Devine Lu Linvega developed the iOS version of Donsol, a game that sees heart suits re-cast as “health potions,” clubs and spades as “monsters”. Starting with four cards, the player gathers health and fights enemies, making their way through an imagined dungeon space making combat calculations -- the health cards versus the monsters. It’s a fascinating idea.
The only problem is that completely unbeknownst to Linvega, someone had already made it.

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“This explains why two people creating a game can stumble across the same mechanic, the same interaction and effect, even if they've never met, never played each other's games,” [Designer and teacher Naomi Clark] continues. She’s even had it happen to her.“It used to make me gnash my teeth, that someone else had also come across the idea that I was so proud of devising, and had beaten me to announcing or launching a game,” she says. “Over time, I've gotten much less attached to the feeling that any game mechanic could truly be ‘my idea’.”

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