I always thought that was from a Shakespearean sonnet, but in fact, it's a Biblical quote (Ecclesiastes 1:9):
What has been will be again
What has been done will be done again;
There is nothing new under the sun.
It's a rather beautiful quote about the repetitive nature of humankind and its history, of the planet, of the solar system. I love cycles - the waxing and waning of the moon (I wrote a poem about it), the entrances and exits of the seasons, even the cyclical nature of the school year.
Considering that the quote is a millennia or two old, the idea behind it is even more true today, now that more time has passed, the earth has continued its regular rotations, humans continue their petty power struggles, and artists keep on creating new ideas out of old themes. The idea that nothing is new is reinforced with internet access to all things amazing and mundane.
If you ever get excited about a turn of phrase that you think you've coined, or an idea that sprung uniquely from your consciousness, before you laugh too hard at your own unparalleled wit and remarkable intellect, I beg you:
DO NOT GOOGLE IT.
You will find that someone has already beaten you to it.
But instead of feeling uncreative and unimaginative, rejoice in the connection that you share with someone else on this planet: the two (or more) of you have had the exact same idea! How cool is that? It's called the Collective Unconscious (thanks, Jung) and it's our commonality we share with everyone on earth, probably expanding exponentially with the invention of digital media. Let's celebrate humanity's interconnectedness for a moment, shall we?
Or, getting back to the cyclical nature of, well, nature, consider this: You may have seen a hundred sunrises. Sure, they're all sunrises and happen at regular intervals, but does that make any of them less special? Less magnificent? Each one is bursting with its own extraordinary light.
So bring on your version of the same old story. Tell it in your own way and rework the nothing-newness into uniqueness. No one will thread string of words together exactly the way you do.
Your voice is still distinctly your own, even if you're singing the same song as someone else.