When clicking on an item in creative when the InventoryClickEvent is cancelled then it duplicates the item. I found this only happens on 1.11. Here is the code I used in the plugin:

    @EventHandler
    public void onInvClick(InventoryClickEvent e) {
        if(e.getCurrentItem().equals(com()) || e.getCurrentItem().equals(bow()) || e.getCurrentItem().equals(arrow())){
            e.setCancelled(true);
        }
    }

When the duplication happens I get this error: http://pastebin.com/ex687HR5

Here is a GIF of it happening: http://imgur.com/a/nvAu3