I am pretty much convinced that EVERYTHING has already been done! After writing Hop! Plop!, a picture book in which a Mouse and an Elephant go on a seesaw, I discovered Just a Little Bit by Ann Tompert, a picture book in which... you guessed it! A mouse and an elephant go on a seesaw. The characters are the same. The setting is the same. The problem is the same. The style and solution are different.
As I mentioned in an earlier post, my first PB manuscript was about a mouse who fell into a hole. Well, I recently read the delightful Help! A Story of Friendship by Holly Keller. Yup! It is about a mouse who falls into a hole and can't get out.
Does it matter that all ideas have been done before? I say a definitive NO. I actually think it is great. What better way to show kids that an idea can be executed in a number of different ways. Fractured Fairy Tales are deliberate retellings of the same old story in a new way. So, don't worry if your idea has been done before. As long as you find a unique way to present it, you are fine. You can't copyright an idea. You can only copyright the expresson of that idea.