PTR
Image: WowheadBlizzard is addressing the issue of NPCs and mobs being briefly duplicated across each layer.
For a brief period of time, NPCs and mobs had multiple copies of themselves, creating some very dangerous areas, or many clones of the same NPC.
Blizzard posted in a server message that this is being worked on after a "blip in the multiverse".
Greetings!. There was a slight blip in the multiverse. We are in the process of de-blipping all zones now.
Source: Wowhead ↗