"If person A WIPOs person B's domain, and person B wins, does that mean that person A can never WIPO that domain again? Does it mean that no one in the world can WIPO person B for that domain ever again?"
Yes and no. Of course a UDRP can be filed, but I guess you are asking about whether a subsequent person would win.
Whether a second challenger would win, or have a good chance of winning, might strongly depend on *why* the registrant won the first time around.
Take this dispute, for example:
http://arbiter.wipo.int/domains/decisions/html/2000/d2000-0625.html
The domain name was cultureclub.com. The Complainant was a nightclub called "Culture Club". The respondent's defense essentially boiled down to "Hey, I never heard of the nightclub. I registered the name because of the pop music band by that name."
The Panel decided in the respondent's favor. They called him a cybersquatter, but they didn't transfer the domain name because they decided the cybersquatting wasn't directed against *this* complainant.
Clearly, if the band then filed a UDRP, they would have a good shot at winning, based on what the panel said in this case.
But, let's take another situation where the respondent wins because the Panel decides the respondent has satisfied the "legitimate interest" condition of the UDRP. Well, if a second dispute were brought, the respondent would make a very strong argument out of, "Hey, the last panel said I had a legitimate interest, and my argument is that I still got one."
That would be an argument with which a panel might well agree - the legitimate interest decision of the first panel essentially saves them some work.
I represented the respondent in the pwc.com dispute:
http://arbiter.wipo.int/domains/decisions/html/2002/d2002-0087.html
As noted in that decision, prior to the dispute the respondent had received communications from Price Waterhouse Coopers, and also from Pratt & Whitney Canada.
There has been only one situation of which I am aware where a domain name was subject to simultaneous UDRP complaints at different UDRP providers. That situation was resolved, so we do not know what would happen if there were simultaneous conflicting UDRP decisions.