Ok, let’s make this clear. It is never OK to put two spaces after a full stop. Period.
My mum does it, my sister does it, my CEO does it, my husband does it. My dog would do it if her smart phone had a space bar (and if I had a dog)!
Farhad Manjoo sums up the history of this niggler in his brilliant post Space Invaders.
The double space harks back to olden times when printing presses were new and shiny (and illlegal) and typesetters were pressed to allow for the uneven width of particular letters.
Those days are over, man!