Its very rare, but sometimes the game creates Extra portals, I think the code first tries to make a new portal somewhere, and then it is like, OH WAIT THERE IS ALREADY A PORTAL and forgets to delete the new one.
The Nether sometime comes with with other portals built already. Once, that was portal under the first one when I first went to that world. Another time, I had to travel 2 kilometers in the light world because a portal popped up way out west in my Cartography map. I couldn't find it in the Nether because it was on a 150 ledge way above a Bright I built over a massive lava river.