If you doubled the prices the customers would think they were being ripped off.
If I was paying for a service and the price doubled, I would immediately look for other quotes. I could accept an increase, but to be asked to pay double, no way.
don't get me wrong, the job may well be worth double, I'm just looking at it from the customers perspective.
My guess is they would cancel, after a few months they will realise that they will have to pay the price if they want their windows cleaned. I doubt if they will give you the satisfaction of paying that price to you though.
"There's nowt as queer as folks". Dai