First we will consider some other instances where individuals have been wrongly credited for the development of an idea, in order to draw some parallels between these other instances and that of Darwin.
An instance of an invention which is often credited to the wrong individual is that of Henry Ford and the automobile. Henry Ford did not invent the automobile. Nor was he the first to mass produce cars; in fact, he wasn't even the first to bring mass production of cars to north America, Oldsmobile was producing and selling cars to the American public before Ford. The automobile is not even an American invention, to the extent which it can be called an invention. It is true that the automobile is an assemblage of many different inventions, and that there are horseless carriages which utilize internal combustion and transmissions, and yet were not cars. Cars as we know them are a specific design for mechanised personal transportation, one of many at the time of its inception. The idea for how cars ought to be laid out was a novel idea, as appose to strapping an engine on to a carriage. It has become an enormously successful design for this purpose, and has proliferated throughout the world. However, the design of the modern automobile was indeed an invention, while it is perhaps not as fundamental as the internal combustion engine, it still an invention. If this invention can be attributed to anyone, it seems that Karl Benz was most instrumental in it development of the form of the modern car.
Not only did Henry Ford not invent the automobile, he also did not invent the other invention credited to him, the assembly line. It took many different individuals working for Ford to create the model T assembly line, and not simply Ford. Moreover, there had existed assembly lines well before that of the model T. In fact Oldsmobile had the first auto assembly line, as well as the first American auto manufacture.
Ford is not the only such misappropriation of credit for an invention, invention of the telephone has been credited to Alexander Gram Bell, when he did not invent the telephone; there had been several working telephone patents before his. Similarly, Thomas Edison did not invent the light bulb. There were patented light bulbs before his, however, Edison did develop a design for a light bulb which was more stable than predecessors and was consequently more successful.
There are many examples of inventions and discoveries which have not been credited to their true source. However, in each of the examples I have presented, the person who is credited was able to popularize the invention or discovery far better than the proper inventors. This is likely why it is so often the case that credit for inventions and discoveries are not ascribed to their true source, for the novel idea gets branded to its populariser, and not its discoverer. It seems as though Darwin has also been misappropriately credited to the theory of evolution, and natural selection in a very similar way as Ford to the assembly line or Edison and the light bulb.
It seems as though Darwin has been unjustly assigned credit for the theory of natural selection. What are the consequences of this for the theory of natural selection? Do light bulbs work any less because they may not have been invented by Edison? Did telephones stop working the moment you first heard that it was not Alexander Gram Bell who invented the phone? Of course the answer to these questions is no. While it is indeed an injustice to the proper discoverers/inventors in these cases of misappropriated credit, and they ought to be celebrated belatedly once it has been discovered. Moreover, I suspect that we will continue to learn of common inventions and discoveries in which the person we ascribe credit for it, is in fact not the actual inventor.
As a result, the issue of whether or not Darwin's work was original has little bearing on the merit of the theories he argues for. It is a very interesting point of history, but not particularly relevant in terms of the strength of the theory itself. The theory of natural selection and sexual selection must be judged on their own merits, and not that of its creator/populariser.