I recently started a new hardcore world in Minecraft after deleting my old one. My old world was at around 87 days. I'm on day 15 in my current hardcore world and I just created my first Nether portal. Upon entering the Nether, I look to the right and see the ladder system to my netherite mine from the old world that I had deleted. Behind me is the exact same Nether fortress as the one from my old world. The entire Nether world is the same as my old one and it includes everything I built in my last world. Including the Nether portal. So now I have two Nether portals in one Nether, one of which was build in a completely different world. After going through the other Nether portal, I arrived in a different area of the map, about 200 blocks from my original location. The overworld is correct and is the way it should be, but the Nether is the one from my old world. I have no idea how this could have happened. I've experienced this before but in a multiplayer server where I forgot to reset the End and the Nether after wiping the overworld. I have a game clip of me ending up in my old nether. The quality is lowered so it would fit here but it gets the point across. If anyone knows how this whole thing could have happened and how I could fix it, please let me know.