Double spaces may occur for a variety of reasons: Perhaps you’ve deleted a word but forgotten to remove the space before it, or maybe you’ve cut and pasted, and some spaces have been included where they shouldn’t be. Some readers may not notice, but to me, double spaces stick out.

Note: When I say ‘double spaces’, I mean between words, not between lines or anything like that. I mean when  you see   something like    this  .

So, how to fix the problem:
Get in the practice of searching for two spaces next to each other when you’ve finished your post.