XP glitch when taking to many items out of a furnace

When I have smelted an item and have a stack of that item already in my hand, and I double left click to add the single item to the stack of items, it correctly doesn't let me add the item to the stack, but still gives me the experience. The glitch is when I can do this multiple times, ending up with 30 levels in only 20 or so times doing this.