I am 100% sure this will not be an issue. As an example, from what I understand, when you post on Reddit you immediately copyright your work. So, if I post a story there, even if it's not mine, I get the copyright to it. There is nothing preventing me from doing that, and Reddit is fine. So we are fine too.
Second, from my understanding, if you don't care about the license of your story (or if you want your story to be CC-BY-SA or similar), you will not have an issue. For example, Reddit gives you the copyright to your story. Then, you post the same story here. This results in the copyright from Reddit being voided and that work will be licensed under CC-BY-SA. If you are OK with that, you are fine.