Wow, I've been searching for a solution like this FOREVER! The tricky part is I want to search for "double-space" but that inevitably pulls up discussions about LINE spacing, not spaces between sentences/after periods.
Oddly, while most themes display two spaces in a row the way they should (as a single space) some display two spaces AS two spaces. The crazy thing of it all is that the source code doesn't show " " as one would expect, but rather 2 spaces " ". So that leads me to think that some themes have a funky CSS that displays " " as double-space.
All ranting aside, that's why this function is so crucial, and probably should be built into the WP core. Perhaps with an option to be disabled in the admin in case the blogger wants to use double space. Just a thought...