> HackerOne should start requiring companies pay researchers for duplicates
That would create a perverse incentive for researchers to tell their friends about the vulnerability so that they can resubmit it and also get a bounty.
The problem could be solved on the side of the researchers by splitting the bounty among all submissions of the same bug, but anyone else with access to the report (employees of either HackerOne or the relevant company) could try to get a share by having someone create a duplicate report.
First come, first served seems like it would be the hardest to game, as the first reporter is guaranteed to have actually done the work (not counting rogue employees who create bugs to "find" and report).
There should probably still be some kind of reward for duplicate reports to avoid discouraging researchers, but something symbolic like publicly acknowledging that they found a bug might be enough to provide validation.