Calculus is a great example of what you questioned us on. We know the two men had similar conclusion with similar math proofs. But even then it is clear that the ideas were derived(pun intended) separately, because while the result was the same, how they got there was not exactly the same.
Such advances are RARE.
The chances two people create the exact same paragraph, EXACTLY is VERY unlikely.
That being said, my daughter, when she was in 7th grade was accused of plagiarism by one of her teachers because her work was not that of a 7th grade girl . Luckily another teacher stood up for her and said was capable beyond her years. No copying needed for the accusation.
So, I take a careful look before accusing one of illicit copying. Gay is not ignorant and therefore it is unreasonable that it was repeatedly made mistake. If it was reasonable excuse, she is incompetent. Those two choices are the real problem here. She did it on purpose or she is incompetent. Either way is a big problem. I don't care which was it falls, as I have not claimed that she plagiarized, only that is a reasonable version. The alternative is not a better choice, but still is viable.
Either outcome results are similar or the same, she isn't the excellence that school deserves.