All nether portals created in the overworld, regardless of distance and location, link only to the first generated portal within the nether, even after destroying both original nether and overworld portals.

Further, creating a new portal inside the nether generates a new portal in the overworld. Stepping back through the new overworld portal disregards the previously built one in the nether and takes you to the first generated portal in the nether, even if previously destroyed. 

Basically no matter what, any and all portals generated or built in the overworld will only link to the first portal generated in the nether. Attempted in multiple different worlds with same result.