It is a fairly common phenomenon for two or more scientists to be developing the same idea at the same time. Sometimes an idea is simply in the air, and several people may be attempting to articulate it. This, rather than an inventor working day and night on a unique idea, is the model of scientific competition.
In that circumstance, it is the one that gets the publicity first who is likely to be remembered and the others forgotten. It was a slower paced competition in Darwin's day compared to today's rush for the front page of the New York Times, but the situation was quite similar.
It makes the quest for scientific prestige a winner take all competition, with one author triumphantly remembered as the discoverer and the others ignored in the evolution of ideas, very unlike the picture we have given of the evolution of species.
But since that kind of competition was prominent in Darwin's thoughts, considering that he could very well have been 'scooped' by Wallace, it is not unreasonable that he chose it as the model for evolution. Nor is it surprising that academics have not questioned the model since, because they are as much immersed in that same dog-eat-dog competition for scientific prestige as Darwin was.
Darwin was co-opted by the generators of racist propaganda because of his own fear that he'd lose prestige to Wallace because science was being played as a winner take all game. But that's not the game that leads to species survival.