I would be more interested in something that doubled the efficiency (output/effort ratio) of work for 10 million people. The focus on money alone is asking for manipulation or even abuse. For one example, if we somehow pushed 10 million students to work full time instead of part time, that could "double" their income -- all the while decreasing the quality of their learning. For another example, if you took a business sector that produced an essential good (like clean water) and simply tripled the price (say by mandate), that could double the income of those working in the sector. For yet another example, if all lenders suddenly conspired to double their interest rates, that would also increase their incomes without actually helping society. To top it off, if you doubled the value of welfare payments, that would also double select persons' "incomes".

Aside from improved efficiency, the idea of giving unemployed persons fair jobs is also a worthwhile consideration. An innovation that allowed disabled persons to work more effectively is another idea. The main reason I focus on efficiency here is because, at least when not monopolised, it is virtually guaranteed to improve the quality of life for humans as a whole. Simply doubling someone's income does not have a guaranteed positive effect on society.