To quote J Michael Straczynski, author of comics, books, and 92 consecutive scripts for the TV series Babylon 5:
"Ideas are a dime a dozen. Given 10 writers the same basic premise, you'll get back 11 very different stories."
He goes on at length to say that the value of an author, or other artist, is not in "ideas", it is in the execution. It is in turning an idea, old or new, it doesn't matter, into a finished piece people want.
The idea of star-crossed lovers was old when Shakespeare wrote Romeo and Juliet. It was older still when they made West Side Story. Both are still great works, because they are more than just the idea.
As an aside - this is why copyright doesn't protect "ideas", but protects your particular expression of those ideas.