I live the Portland, OR area, so let's use a famous local institution as an example: Voodoo Doughnuts. Their base product is a doughnut. They haven't created some revolutionary new form of dessert that combines recycled rubber mulch with fried pine cones and used motor oil. No, it's a flippin' doughnut. Everyone makes and sells doughnuts. Have been for many, many years. The difference is in what the folks at Voodoo do WITH that doughnut, from the unique toppings (grape dust?) to their public image.
Like the doughnut, there's nothing new about the Hero's Journey archetype. But the reason why they both endure is that they work. Even variations on the doughnut follow the same basic recipe. A snack made out of recycled rubber mulch probably wouldn't be very enjoyable. So if you love three-chord punk-pop music or cheesy romance novels, don't worry that they've both been done before. If that's what inspires you, follow your gut, and then sprinkle your own grape dust on top of it. (You can always make fried pine cones with your side project.)