I often find my inventions are things that already exist. One time I thought of a brilliant mode of transportation - a train that lays down and picks up its own tracks. Turns out that's known as a tank.
My latest re-invention was driven by rejection, an important lesson everyone needs to overcome and master, the sooner the better. So the idea was a website called shiton.it, where you can post anything, ANYTHING. Your shitty poem. A picture of your shitty child/puppy/kitten/sugar glider. Your shitty food porn. Post anything you're proud of and just let the internet verbally shit on it.
Turns out, that's called 4chan.