If you do that, you're merely trading one grievance for another: "evil company marked my bug as duplicate to avoid paying" for "evil company claimed to have gotten duplicate reports to weasel out of paying the full amount". More people upset, although individually, maybe to a lesser extent.

The core issue is not the reward division algorithm, it's the inherent lack of visibility. One solution here would be to just open all reports after a while, but this creates problems of its own. One is that it gives ammo to people engaging in dishonest or clueless PR. Another is that some researchers don't actually want visibility, because their employers have murky rules around such engagements, or because they have some far-off disclosure timeline in mind (as a part of a presentation at a conference, or whatnot).