Nether Portals don't link properly between the Overworld and the Nether

When I play Minecraft with a friend who has joined my world and he makes a nether portal it links up to the corresponding portal in the nether but when I make a portal of my own in the same world my portal teleports me to his portal in the nether then if I try to go out of my portal it spawns me back at his overworld location, not mine so sadly I would have to go all the way back to my base to then use my portal again. long story short my portal doesn't link to a corresponding portal in the nether but instead linked to my friends corresponding portal in the nether. please fix