I'm drawn to the subjectivist option. (Plausibly, if anything is subjective, pleasure and pain most surely are.) What matters is subjective suffering, not the objective qualities of pain. It just turns out to be a curious fact about human psychology that you can make us suffer less by inflicting additional (if attenuated) pain.
Parfit seems to assume the contrary view in arguing against temporal neutrality in Chp 8 of Reasons and Persons. In his 'Case Two', you wake up the day after a painful operation, though you cannot remember exactly how long it was. A nurse tells you there are two possibilities: (1) You had 5 hrs of pain, but the operation is now over; or (2) You had just 2 hrs of pain yesterday, and have another hour still to come. Parfit suggests that the first seems preferable, despite being worse for your life as a whole. But, it seems to me, one episode of extended pain may have a roughly constant disvalue no matter its actual duration, at least if you cannot subjectively tell the difference. If this is so, then the first option is actually better for your life as a whole. It contains merely one episode of pain, whereas the alternative contains two.
One objection to my position is that memory flaws may distort our retrospective understanding of how bad a pain really was at the time. (This seems to be what Daniel Gilbert thinks of the attenuated pain case, based on his remarks in a talk last week.) I'm not sure what to make of this suggestion. But even if we're drawn to a more objective theory of hedonistic evaluation, we may only wish to count as distinct experiences those that are qualitatively distinct. (We would then think that duplicate universes, or Nietzschean eternal recurrence, would make no difference to the value of the world.) Most of the time, even intrinsically identical pains are embedded in discernibly different experiences, and so count as recognizably distinct. But in Parfit's hospital case, it seems like the duration doesn't introduce sufficient qualitative differences. After a while, many moments of hospitalized agony all blur together, and we may think the reason for this is precisely that there is truly nothing in the experiences to distinguish them. And so they count for just one.
My intuitions on these cases are all over the place, so I'd love to hear what others think. For any who prefer a more practical example, consider Michael Vassar's past comment:
One potentially important example of experiences that may be identical enough not to stack and may not be comes from factory farms. It's plausible that factory farms aren't all that bad, but also plausible that they are good candidates for "worst thing ever". I'd definitely like to know which is true.
What do you think? Is it true that (a) qualitatively (sufficiently) identical experiences only count for one? and (b) many animal pains are (sufficiently) qualitatively identical?