Unlike the other guidelines, you can almost certainly ignore this . In the rare case that your story is accepted at two publications, just call or write all the other magazines considering your story and tell them not to bother. You probably won't encounter too much resentment. We recommend that you send to 10 to 20 magazines. It's very important to look at the publication at least once before you send it in.
Editors like to say that the number one thing they look for is familiarity with their magazine.
Finally, we recommend that you remain realistic, but still only submit to publications that you respect. It's better to not publish a story at all than to see it in a crappy publication.