Personally I never try to poach anyone's work, but if you don't knock on doors then you'll never get any work, if the customer says they have a windy, I just say ok thank you, I don't ask them if they'r happy with them or anything, I just go to the next house.
On one cul de sac I got 5 new customers next door to each other then got to the end of the street and was told a windy does the whole cul de sac, so if if have been there first and believed them, I wouldn't have got those customers