As a prank, aliens come to earth and double our population. This new people are integrated, with false records, and the aliens messed with our heads so we can't reliably tell old people from new people. No one can tell if they themselves are old or new, since the new people have false memories. The aliens weren't perfect though, and with detailed investigations we can tell people apart if we wanted to, but we probably couldn't do it on a massive scale. Relative population densities and all demographics are the same.
What are the short and long term consequences?