It's a good question (not about anything actually IMPORTANT, but still, a good question). I have had that happen to me once. I've only neg-repped maybe 3 times ever, total. But one guy didn't like it and so returned fire almost immediately.
The problem is, how to draw the line between someone giving out neg rep for legitimate reasons and one giving it out SOLELY for revenge??
What if the initial negative rep is given for bogus reasons? If someone gives me neg rep just because they strongly disagree w/ me about something, or they think I'm a dick because of my viewpoint, would a revenge neg-rep be warranted or justified? I'd say yes - in THAT particular kind of case.
Maybe, if revenge neg-rep is a serious problem (and I don't think it is), perhaps there should be some "cooling-off period" after the initial neg-rep is issued, where neither party can rep the other negatively. That way, only the worst of the worst a-holes will hold a long-term grudge about it and still give neg rep, but if one has truly been wronged, then maybe it makes sense to mark one's calendar and neg rep he/she who wrongly neg-repped you in the first place... I dunno.
It's just rep. Nice to have (for some), but ultimately of little consequence, right?