Don't you hate it when you […] think of a cool story idea, but then you think "No, that story idea is so obvious, I'm sure it's been written 20 times already"?

But even if it has been written 20 times before, you haven't written it 20 times before, and there are certainly people who aren't interested in Sherlock fic, or post-Reichenbach fic, who would be interested in a fic you wrote on those subjects just because you wrote it.

Repetition is not an actual problem in fandom!