Placing an item frame on a chunk border with a comparator attached causes hundreds of the item and the Item Frame to appear. If any other entities are nearby, they get duplicated too. I've tested on two different computers that are completely different.

Works on both Flat world and normal worlds.
Works in both Creative and Survival gamemodes.

Steps to duplicate:
1) Move to a corner of a chunk so your X and Z coordinates within the chunk are 15, 15
2) Face East.
3) Place two blocks under you, and one on the ground behind you to make an L shape.
4) Place a comparator on top of that block.
5) Place an item frame on the front of the top block.
6) Put an item inside
7) Save and Quit
8) Reload the world

Picture 1: I'm at 15, 15 in the chunk I'm in.
Picture 2: The tall part of the L is where I was standing in the first picture
Picture 3: Comparator and item frame, with a Diamond Sword in it
Picture 4: I spawn a Chicken to show that it duplicates all entities.
Picture 5: I reloaded the world and ZOMG CHICKENS AND SWORDS
Picture 6: Showing the swords are real, I picked 40 of them up.