Isn't this idea similar to forbidding selfvotes? If you can't do it with one account, you can do it with another one or multiple ones.
Moreover, how would you define the cap threshold? Is the cap of HP towards one proposal?
Would splitting the total HP usable for proposal voting be better utilized if it was split amongst all voted proposals in some ratio?
Could using logarithmic vote scale be a better way of dealing with similar issues?
Sorry for the unanswered questions. However, I feel I see people reacting similarly but didn't see any reasonable solution backed by more than opinions.
One account one vote would actually makes some sense as it would prevent whales from voting in multiple proposals at a time, at least not with their entire stake. Sock puppets wouldn't help much in this case either as their only benefit would be the ability of voting for multiple proposals but again, not with their entire stake but rather only a fraction of it.