I know the routine for entering a zip and having it "find"
the appropriate city and state from a separate table.  
That's the easy part.

But how do you handle the fact that some zip codes are
assigned to more than one city?  For instance, Amherst,
Pelham, Cushman and South Amherst, Mass. all have the same
zip code (i.e., 01002).

So, if I'm entering a person's data and plug in "01002"
for his zip code, but it finds the wrong city, what are my
options?

Suggestions?