Back in the dark ages, when I was first learning to type, the Word of God as passed down from on high by the instructor (who was more interested in training 80-words-a-minute secretaries than in teaching the rudiments of touch typing to a future English major) was that you double-spaced following a period.
I never became an 80-words-a-minute typist, but those two spaces after the period were hardwired into my brain, not to mention into my spacebar-hitting thumb.
Cue the musical montage representing the passage of time, with the tappity-tappity-tappity-bing! of the typewriter fading into the musical-popcorn boop-boop-boop of the old computer keyboards, and that sound fading in turn into the near-silence of keyboards today . . . followed by the Word of God saying that it is now customary to space only once after a period.
Why is this something it doesn’t pay to worry about? Because, one, of all the reasons an editor may have for rejecting your manuscript, the question of how many spaces you’ve put after your periods is way low on the list. And, two, if the whole thing bothers you that much, you don’t have to sweat blood retraining your spacebar thumb — all you have to do is run a search and replace during the final edit, and change every instance of two spaces to a single space instead.