What comes to mind when I read the word Unique is the question of whether or not anything truly is.
Unique is defined as 'being the only one of its kind'.
Surely no one on this Earth or in this universe is exactly like you, or me for that matter, and therefore by definition of the word that is exactly what each and every one of us are. Unique.
At the same time, Unique is also defined as 'unlike anything else'. That is a very different situation altogether. Is there really no one in the world like me? No 50 year old, long time gamer geeks with a passion for Science Fiction, Superheroes, and the Muppets exist other than myself? I find that very hard to believe.
When I create campaigns, characters, even game systems, I like to believe them to be special, not your average fare, maybe even a bit unusual. I'm sure I'm not alone. At the same time, I have never thought of what I do are truly Unique. The ideas came from somewhere, sparked by thousands upon thousands of influences I am both conscious and unconscious of. Often my creations are directly inspired by a popular idea I want to do differently, or one that failed to work but I see a way to 'fix' it.
I don't run Dungeons & Dragons, surely the most common and mundane of all RPGs, so my games must, at least by comparison, be progressive, innovative, even avant garde! Well, no. They are not, by default, any more or less singularly revolutionary then the millions of D&D games being run across the globe.
Remember, I tend to run IP games quite often. I love running Star Trek and Star Wars for example, and I am most assuredly not the only one. Beyond that, I did not create either of those settings myself. I am, at best, a highly skilled and dedicated hack, with a real passion for those franchises.
Is anything in gaming Unique? Are there games with settings, concepts, characters, and/or rule mechanics not seen in any other game? I'm sure there are. I'm sure if I thought long and hard enough I'd come up with a few. Honestly though, I don't think that's what really matters.
Being Unique isn't as important to a successful and memorable game or as being Entertaining. Whatever it is that makes your games work, whatever causes players to pester their Gamemaster as to when the next session is, that's what matters. It could be the same old Fighters and Clerics in the same old Dungeon. It might be yet another alien planet visited on the ongoing mission of your starship. If it is what you enjoy, what gets you and your friends excited, then it doesn't matter that large numbers of others are doing roughly the same thing.
Even if they are, only your group is running it with your friends, in the way that you all love. And in that way, even the umpteenth delving into the Caves of Chaos below The Keep on the Borderlands is absolutely Unique.