When I was in the Nether, I placed a Nether Portal Block to get out, and it took me to another world.
I wanted it to either take me to my home portal block, or make a new portal and I just use my compass to lead me home.
I thought it would make another portal in the same world, but I don't think it did.

I was playing in a superflat world with overworld (I think, but it could be tunnelers) preset. I have been building and exploring for a long time, and built some cool things that took a long time to make. I placed a nether portal block, not built a portal, in the overworld. I went into the nether, and played around a while with gravel, wither skeletons, blazes, and such. I placed a nether portal block, but didn't use it. I placed a nether portal block in a pillar of a nether fortress, a long ways away from my original portal, to get home, and it made a new portal in the middle of superflat nowhere. I got out my compass and flew to where my spawn point should have been, but it wasn't. My spawn point was, before, just outside of the village that my castle was next to and connected to. The spawnpoint my compass took me to was nowhere near my village. I typed /kill and died, and popped up in my spawnpoint, the one from the first world. I went back into my portal block and went back to the fortress I was in. I placed a portal block in a different place, near the fortress where the other portal block was. I don't know if it took me to the same portal in the middle of nowhere as before, but I followed my compass to the same effect as before. I typed /kill again, and died, and popped up in a spawnpoint that looked a little like my spawn point in the regular world, but my village and castle, along with anything from the second world where I typed /kill, were nonexistent. I placed a nether portal block and it took me to the same portal as the portal I come out of coming from my house portal block. I went back through the portal and I was in my house.

If you want to reproduce, try going to a superflat world, placing a single nether portal block, going a long ways away to a nether fortress, placing a single nether portal block, going through it, and following your compass. I don't know if it can be replicated, but if it can, you should try that.