It's closer to tripling your population. Which is impossible if your fertility rate's below replacement... well, unless you import enough people that those imported outnumber those already there by 2 to 1.

I first learned of it here on HN, someone submitted it last year.

>The economy can handle it over that time frame.

What economy? Are you planning on finding jobs for tens of millions more people? One might say that you don't have enough jobs to go around as it is.