Peeke (Joe),
It's a pain when this happens, but it does happen.
If you're hard-nosed, charge triple (at least) and clean all the paint off in one go.
If you're not tough enough (like me), explain to the customer that to remove all that paint from the windows isn't in the price that you quoted.
However, you'll remove the paint from one window per visit; (and start with the prominent windows like the kitchen or living room).
If you don't use ladders, just remove it from the ground floor.