Basically the change event for a combo box is fired twice whenever the combo has an initial value. The combobox unsets it's value for the deselection of the dropdown and the selects the chosen value. But since it's doing these as two operations, there are two change events. One from oldValue -> null and one from null -> newValue.
So from the standpoint of the user selecting a new value from a combobox that has an initial value, there is no change event that has oldValue -> newValue.