MediaWiki:Gwtoolset-reupload-media-explanation/en

This checkbox allows you to re-upload media for an item that has already been uploaded to the wiki. If the item already exists because of one of your previous uploads, an additional media file will be added to the wiki. If someone else has already uploaded a new version of the media file, then your new upload will be ignored. If the media file does not yet exist in the wiki, it will be uploaded whether this checkbox is checked or not.