On the Javadocs this event says

"The item that's breaking will exist in the inventory with a stack size of 0. After the event, the item's durability will be reset to 0."

but this isn't exactly accurate and I suggest it should say something like

"After this event, the item's amount will be set to (item amount)-1 and its durability will be reset to 0"

The current text sounds like in the event the amount of the items is 0 and you should be able to set its amount to 1 to cancel this event but this is not the case and you need to do event.getBrokenItem().setAmount(2); since after the event it takes 1 away from the amount.