Probably this is becoming most clichéd question :)
Generating a totally unique idea and selling it to people will probably get you another "Apple"(I don't mean it literally). But till the time it does not happen, there is nothing wrong with working on something which is shared by someone else too, its just that yours should be better than others and should have enough differentiators to "rock". I am not going to quote "Google", "Facebook" et al. here again because everyone already knows the story.
Generating a totally unique idea and selling it to people will probably get you another "Apple"(I don't mean it literally). But till the time it does not happen, there is nothing wrong with working on something which is shared by someone else too, its just that yours should be better than others and should have enough differentiators to "rock". I am not going to quote "Google", "Facebook" et al. here again because everyone already knows the story.