IMHO, one space is correct for manuscripts. The two spaces came from a previous era when we used typewriters and dot matrix printers and nonproportional fonts and two spaces after a period made the documents much easier to read. Nowadays proportional fonts will adjust the space after the period anyway, making it unnecessary to use two spaces. While wikipedia isn't the definitive authority, it does explain it well here.
And here's the MLA ruling on the issue - one space. http://www.mla.org/style_faq3
Having said that, there is still some controversy over the issue. A recent poll by literary agent Nathan Bransford found only a slight majority of his readers in the one space camp:
In a minority view, the APA publication manual blog wants two spaces, but it recognizes that:
. . .the usual convention for published works remains one space after each period, and indeed the decision regarding whether to include one space or two rests, in the end, with the publication designer. . . .
So, I'm in the one space camp.
If you've put two spaces in, it's easy to do a universal find and replace. Just search for ".[space][space]" and replace it with ".[space]" It might take a little while, however for those of you (like myself) who learned to automatically hit the space bar twice after a period, to retrain yourselves to only hit it once. It did me anyway.