It is so "obvious" that it would be very easy to "steal" the idea and not give credit to Marco. After all, you could always say you had the idea independently.
So I'm not really sure what point you are trying to make.
I don't agree that it's impossible to know if an idea is original or not. For instance, on one extreme, if 2 composers published a very similar concerto at about the same time, it would be assumed one of them plagiarized the other. On the other extreme, if you're working on an arithmetic problem and there's only one answer, then of course no one would be accused of plagiarism.