This is a bug that can copy the Item in the inventory. After my friend tried to logged off to avoid the creeper exploded. however he still got killed even he Is not exist in the world. all of his item was dropped off.

After he logged back he found that the item still in his inventory. Then he used this rapidly to copy the stuff such as enchanted item and diamond. And now he have a 2 stack diamond block and full diamond stuff.

This is the bug I found in the latest snapshot in 1.9. I hope the bug can be fixed until the 1.9 is release. Thank you.