UploadWizard stores uploaded files in the upload stash, then proceeds to collect metadata for them from the user. That sometimes breaks and the upload process has to be restarted. When that happens, there is no reason why the user should have to reupload the file (we could be talking about huge video files) which is already sitting there in the stash.
One possible implementation: in the upload step, if there are files in the stash, show a warning to the user saying "there are aborted uploads, do you wish to continue them?" and then allow him to pick files from the stash. (This does not work for Flickr uploads where the URL is a metadata source as well, although not sure if the stash is used at all for those.)
See also T77826.