It is a personal pet peeve of mine when people double-space after a period.
Yeah, I know, there are more important things to worry about but this one thing drives me crazy.
From Wikipedia's entry on "sentence spacing" (also, I know, Wikipedia... right. This is a long sample. I suggest you skim):
James Felici, author of the Complete Manual of Typography, says that the topic of sentence spacing is "the debate that refuses to die ... In all my years of writing about type, it's still the question I hear most often, and a search of the web will find threads galore on the subject". This subject is still widely debated today because many typists were taught to use double sentence spacing in school. As a result, there is a common misconception that double sentence spacing is "correct", even given modern technology and proportional fonts. This is similar to other obsolete typewriter conventions, practiced in deference to its "severe technical limitations", that are still used by writers. These include the use of prime marks (or "dumb quotes") for quotation marks, underlining words in place of italics, and using hyphens to approximate en and em dashes.
Many people are opposed to single sentence spacing for various reasons. Some state that the habit of double spacing is too deeply ingrained to change. Many claim that additional space between sentences makes text "look better" or easier to read. Proponents of double sentence-spacing also state that some publishers may still require double spaced manuscript submissions from authors. A key example noted is the screenwriting industry's monospaced "standard" for screenplay manuscripts—Courier, 12-point font—although some works on screenwriting indicate that proportional fonts may be used. Finally, although some reliable sources state simply that writers should follow their particular style guide, proponents of double-spacing caution that publisher's guidance takes precedence, including those that ask for double sentence-spaced manuscripts.
In opposition to these ideas, many experts state that double sentence spacing was only relevant when faced with the limitations of the typewriter, and is now obsolete for most uses—especially given the capabilities of modern computers and digital fonts. Although typewriter users had only two choices (to strike the space bar once or twice), modern proportional fonts allow users to manually adjust sentence spacing to thousandths of an inch for visually pleasing typesetting. However, it is acceptable even for monospaced fonts to be single spaced today. Another consideration is that, since terminal punctuation marks the end of a sentence and additional spacing is itself punctuation, additional spacing is redundant.
Some assert that, because the double-space typewriter convention is still being taught widely in school, students will later be forced to relearn how to type. Although a small number of style guides in the United States allow double sentence-spacing for draft work, no known style or language guide indicates that double-sentence spacing is proper for final or published work today—and many state that it is incorrect. Publishers usually require manuscripts to be submitted as they will appear in publication—single sentence-spaced. Many writing sources recommend that prospective authors remove extra spaces before submitting manuscripts, although publishers will use software to remove the spaces before final publication. Finally, some experts state that, while double spacing sentences in unpublished papers and informal use (such as e-mail) might be fine, double sentence spacing in desktop-published (DTP) works will make the final result look "unprofessional" and "foolish".
I learned to type in 1988 or '89 in a class where my work was routinely posted on the wall with all the other perfect work (I think it's very funny that the teacher never noticed that interspersed with all the examples from the book that I'd typed correctly, I almost always included the lyrics to whatever Cure or R.E.M. song was taking up space in my brain. Rock of the 80s!). We've previously established that for the most part I love to read and to learn but I'm not a big fan of school. I tend to fall asleep in classroom situations; I can't help myself. My typing class was a big loud room full of like-minded bored students, and I, for some reason, excelled. Maybe it reminded me of the noisy band room. Whatever. We were taught to double-space at the end of a period because we were typing on 50 year old machines that always needed a new ribbon. They were called "typewriters."
Years later, in 1997 I took a job working for an optometrist in Sherman Oaks. I'd kept up my typing skills thanks to an (ex) fiance who was studying for his Master's degree in Sociology, and his typing skills weren't as good as mine so I did a lot it for him. It was while typing his papers that I learned of the lack of a need to double-space at the end of a sentence. (I also did some of my own college work, albeit, much less enthusiastically.) Anyway, working for Dr. Pearl (no, not that Dr. Pearle) required me to type a lot, and I did it uncomplainingly, though I'd since broken up with that guy. Still, there was no extraneous spacing going on and we were all happy. Well, maybe not my ex.
Then I had to leave that job when Dr. Pearl sold his practice, and I went back to the world of retail (where my manager hand drew his sales reports) and my typing was limited to inter-store email, which were always formatted correctly, though the subject matter was usually silly.
When I got my County job, I had to type again, and a lot, and I found many people still clutching to their double-space at the end of a sentence habit. I ignore it when I can, but sometimes, if I get to edit something, I get rid of them, quietly, with little fanfare or griping.
It wasn't hard for me to quit doing it. I think it took less than a day to get over it. You can too. You are not a typesetter, there is no need for it. Let it go. Let it go. Let it go.