Issue:

Nether portals can link to each other even though they should'nt.

A nether portal at x 1000 z 0 can sync to a portal in the nether at x 0 y 0.

This can't be intended because this allows players to travel thousands of blocks in no-time.

To reproduce:

Create a new world.

Go to x 0 z 0 in the overworld and create a portal.

Go through the portal then back.

Go and build a nether portal at x 1000 z 0 

Go through it and see that its the same nether coords as the first portal created.