Not double, actually, because part of the point of a basic income is to replace much of what we currently think of as welfare. For example, Social Security as we know it could simply vanish, its function completely replaced by the basic income. In theory, salaries for government employees of all types could also be reduced by an amount equal to the basic income, leading to no change in take-home pay but saving money within the programs which employ them.

Nevertheless, as you point out, it would necessarily represent a massive expansion in federal spending: on the order of 50% or more.