For the most part, it’s a couple of reasons. First, when stories are accepted into a publication, they get posted according to the publish date. So if you submit a piece that was already published, it basically disappears down the feed.

The second reason is that often people publish stuff that kind of needed another edit or two, and when it doesn’t do well, they try to “bump” it by submitting to a publication. Not always, but often enough that most publications just said no more.

My experience has been that it’s always best to err on the side of caution and submit a draft. Better odds and more visibility. :)