What has been will be again,
what has been done will be done again;
there is nothing new under the sun.
A common misinterpretation of this bible verse is that all of the good ideas have been thought up already.
This is pure bullshit. It's been bullshit for millennia, since long before Ecclesiastes was written, and it will be bullshit long after.
Everything you write is new.
Even if you borrow, copy or even outright steal ideas, you'll still combine, illustrate and articulate them using your unique voice (this excludes plagiarism, obviously). The fear that we'll only think up unoriginal ideas is just another fear that keeps us from writing.
The real crime is letting this fear stop you from putting your work out there.