That's a new one to me. Are you using Safari as your browser. I
found this at Wordpress support though it seems to only be Safari specific.
I realize this would be an extra step, but you could do a quick search and replace in Word or in a text editor to convert double spaces into single spaces before bringing the post into WP.
Another thought is to try the other view when publishing. If you use visual view try the html view, if you use html view try visual view. I'm not sure that will do anything, but it's worth a try and I've seen it solve other things.
I'll keep my eyes open about this and let you know if I find anything.