my experience has been that goto is very good about filtering out these "repeat" visitors. they use several techniques, many of which you can figure out if you're a little technical and you pay attention. however, they wont disclose these techniques, in the interest of avoiding educating troublemakers. fair enough, i say.
we do a *lot* of advertising for various clients on goto, and comparing our logs vs. their billed expenses leads me to believe goto makes solid efforts at eliminating "spam" clicks.
we filter out the spam in our logs, and the variation i've seen between ours and their billed clicks as recent as last month, for one account with 30k+ visitors, was ~2%. i think that's pretty darn good. you get much worse stat variation in banners clickthroughs, in my experience.
others' mileage may vary.