When someone joins my map on my android tablet and I kill them or they kill me whatever is in my hand will duplicate and say 99+ or sometimes just go into stacks of 64. This also happens when someone is in the game and when they quit and rejoin whatever was in their hand will be duplicated. Every item that I have tried duplicating has worked things like food and blocks go into stacks of 64 and weapons, armor, and minecarts go into stacks of 99+.