Things apearing in world not in the place i built them

A structure made of of slabs a walk way has dupelicated its self destoryed terrain and ploped its alternate self down 20+ bocks from its previous position, and it has destroyed a large chunk of a mountain
that is all

What I expected to happen was...:
Describe what you thought should happen here

What actually happened was...:
Describe what happened here

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...
3. ...