Originality is Debatable when No One Can Own an Idea
How many times has this happened to you? You've been jotting down ideas to include in your manuscript when you see one of the exact same ideas carried out in a finished work. You don't know this person and they have no way of knowing you, so neither one of you stole it. Suddenly your idea doesn't seem so original anymore, and you begin to doubt yourself and whether or not you should include any of these ideas at all. However, just because that person put it out there first does not mean he or she owns the sole rights to that idea; neither of you can really own a single idea, just an overall concept that is about as unique as the next one.
Being on the same wavelength just happens to be the case at times. Bogus claims of copyright infringement happen to writers too, except for the fact that those who file them are paranoid or just ignorant of this concept. It's easy to be so caught up in a wavelength that you are unaware of anyone else riding it. Therefore, while there are real cases of rip-offs and theivery out there that dance the fine line between parody and plagiarism, most cases are simply between those who happened to have the same idea independently of each other. In fashion, as evidenced on Project Runway, the designers are expected to have done research on all the famous designers so that they don't unwittingly copy an already classic design (some still don't and are made to suffer for it). If writers were expected to do this, they would never get anything done because there's too much material out there to cover, a lot of it very obscure.
Depending on how integral the idea is to your story, you can either make adjustments or keep it in as is. Commonalities do occur and there are very few who would seriously take offense to them. For example, the naming of a pair of characters who are siblings (especially twins) can either vary widely or appear several times in different works. Whether they are throw-away characters or main ones, what makes them unique to any story will always be found in their personalities rather than in name. Another example would be a character with a certain personality trait that clashes with some aspect of their assigned duties. Depending on what those are, even a clash of the same elements can only be chalked up to coincidence at best.
There is a collective consciousness in the world that influences us all on a psychological and cultural level. Sometimes we are directly inspired by what we see, and at other times inspiration seems to pop into our heads from out of the blue. While this may not stand up in court, the burden of proof is on those accusing you of theft if it ever comes to that. Anyone who tries to build a case around ownership of ideas that anyone could have is just fooling him- or herself. There is no reason why both of you cannot use the same idea, especially if it serves a slightly different purpose within each of your works. With seven billion or so people on the planet and trillions more throughout history, there is really no such thing as true originality anymore. Just ask Hollywood, which has turned out almost nothing but rehashed storylines, reboots, and remakes (which aren't always bad things, but it's all you see nowadays along with unnecessary sequels).