Quoting from the
General Book Uploads Posting Guidelines:
4. If you are considering creating a particular eBook, please check before you start whether or not that book has already been uploaded to MobileRead (this can be done by clicking the "E-Books" link on the blue bar near the top of the screen, selecting the "Browse Latest Uploads" option, then selecting the "Search E-Books" link and entering either the title or the author into the search box). If you find that the book has already been uploaded, you are very welcome to upload a new version provided that it "adds" something different to the version already present - this might be a better layout, a new format, significant proof-reading and correction, proper dashes or "curly quotes", for example. If this is the case, when you upload your new version, please add a link to the earlier version (or simply say who the uploader was), and state in the description of your book how it differs from the earlier version.
Now, that's what the guidelines say, but I would say it is often ignored and almost never enforced. If you think it is worth spending your time in creating a new version of an existing book, this probably means your new version "adds" something.