Villager trading dupe

I'm not too sure if it's only with a stack of emeralds as we have not tested it much on our server and I have tested this in single player. If you trade a villager and put the emeralds in, what you want to do next is shift click the items (not the emeralds) and hit e at the same time you do this. If done correctly, you will get the emeralds and a stack of the item.