goto chunkborder chunk number 15 and bil a L shape with item frame on 15
make sure comparitor is behind block with frame it needs to face south or face east to work

here is a youtube video
https://www.youtube.com/watch?v=mXBDcQP-U7E

explains everything and how to do it , this will dupe all entities in the chunk affected and create 100s for each 1 have 1000 cows end up with 400,000 cows

as this crashes servers this needs to be fixed before 1.8
same with alot of dupe other i dont care about as they dont cause crashes