We should always try to be original and creative, but the truth is, most things have been done before. There are already stories about twins in the world. There are tons of stories with magical elements.
What makes the biggest difference is the new combinations of these old things. The depth of your characters, the excitement and emotion of the story. If you can take me out of my regular life and bring me into a new world, you’re doing something right.
Plus, your story will be new to someone. The more time passes, the more creative work there is in the world. But no one is going to experience all of that creative work. Personally, I’m not a big movie watcher, so when someone says, “I think this is a cool idea, but is it too much like that one movie?” Normally my response is “I haven’t seen that movie. But the idea is pretty cool.”
I also think there are some stories that should be told multiple times.
For example, I've read a lot of stories similar to Dawn’s Acapella. Dawn's Acapella is a story about a group of high school kids who put together their own club based on doing good deeds. The main message behind it is simply how a small kind act can have a huge impact on someone’s life.
Now, this message is fairly common. I’ve seen it in brief stories just scrolling through facebook. I haven’t heard the exact same story, but the same general theme.
No way did this stop me from writing “Dawn’s Acapella.” I think this message is so important, it should be told over and over again.
If your following your golden rules, and reading a lot, you'll start seeing a lot of similar elements between the books you read and your own work. You'll also notice how many books have elements in common with each other. Take note of it. Change what you feel you need to. But never give up just because you see a few similar things.
You might have a magical object of power, but you can still set yourself apart from Lord of the Rings. You might have a school of magic, but you can still set yourself apart from Harry Potter. You might have a strong female character starting a revolution in a dystopian world, but you can still set yourself apart from The Hunger Games.
You’ll usually know if you’re going too far. When it doubt, get feedback. If you find yourself emanating a particular author, or you see too many elements of a particular story showing up in your work, that’s when you reconsider. But I’ve known a lot of writers who get slowed down just by one or two little things that “have been done before” and then start questioning their entire creative process.
Don’t doubt yourself, just get going.