We're all working with the same finite amount of resources. There's only so many different permutations of colors, shapes, and patterns that can exist. No one is secretly holding on to a game changer; an unknown design element. As a result, that great idea you've been sitting on? There’s a good chance someone has come up with something similar.
There's a difference between something being a derivative, something being inspired, and a blatant ripoff. It's a fine line to straddle. Don't let similarities deter you from a good idea. Also, don't dress yourself like everyone else and expect to stand out.
If the color red gives off the feeling of urgency that you're looking for, use it! If a skull communicates fully that idea of dread that you want to inspire, go for it! Just don't forget to set yourself apart.
In a video called "It Doesn't Have to be Unique," from the Lynda.com course "Graphic Design: Logo Design Tips and Tricks," John McWade talks about the similarities between the logos for household brand names such as Volvo, Honda, and Sony.
This phenomenon isn't unique to just logos. Everywhere you look, everything you hear, it came from somewhere else. "Everything is a Remix," a video series by Kirby Ferguson, breaks down the way ideas build on previous ideas, making something new and fresh. You'll be surprised how many things are creative descendants of other things. Originality and creativity are not what we once thought.
"There's nothing new under the sun." You already knew that.