Nether portals spawn a new portal even if it is still active

Recently, I was playing in a survival world. I just created my first nether portal and entered it. Then, when I came back out, I ended up in a cave at Y=-15. I eventually destroyed that portal for fear of coming back again. I came out, did a few things, and entered the Nether again. However, when I came out again, another portal appeared, this time high above the ocean.

I look forward to fixing this issue