v. As long as you have proof that you wrote them first, you should be okay. If not, then they could easily report you for plagiarizing them, and without proof... well, you'd probably be the one banned.
The only problem with that is my computer I had them on crashed. I no longer have the original files only the word and dates from the teachers I submitted them to.
If they're still posting their (your) stories here, then starting to double post is not a good idea. I'd suggest speaking with staff and providing them with proof that you did indeed write the story first, otherwise you could end up being the one banned.
Report them. If you can, screen-cap the info from the original Word (or whatever processor you use) documents that you have for your stories - if the create date for them exists before they posted the first chapters on Mibba, you can prove that they stole them that way.