On my Sony xperia z1 the Down event is also fired two times.
What exactly do you expect from the Down event ?
I don't think that it will fire 'permanently'.
In my mind it should fire once when the Button is pressed and when the Button is released Button_Up fires once.
If you want the Button to fire in time intervals as long as the Button is pressed you could do it with a Timer.
Enable the Timer in Button_Down and disable it in Button_Up and call your routine in Timer_Tick.
From what I've seen is that when you press the button Button_Down is fired, then Button_Up is fired and when you leave the button pressed Button_Down is fired a seond time and Button_Up is fired again when you release the button.