I disagree with your premise that doubling the number of dollars everyone has would simply double prices. If you take someone that has $1 million in the bank, and magically make that $2 million, they almost certainly won't double all of their expenditures (they probably wouldn't change what they spend at all on a wide variety of categories - food or clothing, for instance). Rising inequality means that lack of demand is actually a big issue, since we're transferring income from people that would readily increase consumption, given more income, to people that will largely sit on that money. Globally speaking, our factories could easily produce more goods without raising prices much, if any. I believe a UBI would effectively be a transfer of wealth from people that have lots of money (but don't spend it) to people that don't have lots of money (but would readily spend more if they had it), but it wouldn't be the zero-sum game that people often assume it would, just by virtue of our productive capacity not being nearly maxed out.