auto chunk cloning / new "mob"

Put the summary of the bug you're having here

What I expected to happen was...:
to make a special no-problem world from a special seed.

What actually happened was...:
i was playing in the world for a while (30 mins) and suddenly everything was glitching and small chunks were cloning themselves. then rabbits were only rabbit legs and some giant unidentified mob came up from the sand.

Steps to Reproduce:
1. enter seed -969535336
2. load world and turn f.o.v to maximum
3. watch the weirdness happen/ fly around the world a while