When I return home from the Nether, a new nether portal is generated in a random place in the middle of an empty field. It's the same place every time, but I never return through the Nether portal I came in through. 

I only have one nether portal in my game, I should both go and return through the same portal, but every single time I return it respawns a new one - which basically gives me unlimited obsidian because all I have to do is break down the portal, go back to the nether, come home, and then break the newly spawned portal. 

Attached image shows my portal I enter. (There is a backing to it, I have removed the entire diorite backside but it doesn't fix the issue.) The second picture, (portal2) is where I return. As you can see, I had come back previously and there's a portal left over from last time. It just keeps generating new ones.