I guess it's a numbers game, not all your customers will ring and ask you to do them again but if you do go back and do it again foc, you could be making a rod for your own back. The Sahara dust events are thankfully rare but only go to reinforce the preconception that cutomers have that "rain =dirty windows". I'm hoping the upside to it will be new enquiries.
Edit.. I would certainly draw a line in the sand (pun intended) date wise and only consider complaints from customers who were actually cleaned yesterday, the day of the dust downpour then take it on an individual basis. Ie, if you were working on one estate for much of the day and you go back to one, then the flood gate could open, or at least cause bad feeling from those you don't redo. Individual properties, then maybe..