In the past incidences there has been almost no overlap at all, members are few and far between and a scenario where someone would not only live close to another member, but use the same service provider is quite rare.
And for the cases that such thing happens; life's a b*tch and then you die. Can't be helped.
EDIT: Should a really large ISP use one unified pool... that would be a problem, but I've never heard of such thing. Pretty much all the larger ones I've seen divide their dynamic IPs into smaller pools that in turn are assigned to certain geographic areas.