Overworld netherportal is duplicated when extinguished within the nether by a ghast explosion.

Relighting the netherportal within the nether returns you to the overworld about 100 blocks away in an identical height/width portal.  Upon return, the original portal was extinguished.

Relighting the original portal allows travel to the nether with no ill side effects.  Returning to the overworld always returns you to the second duplicate portal.