don't over-complicate things. This is most-likely not a bug. Chances are that you are just downloading the same spam file with different titles and/or you keep connecting to the same bad peer, or group of peers. This is why the file does not verify. The file does not get moved to your downloads folder, therefore it is not removed from your search results. Even if it does, it will still show up as being on your downloads list unless you close that search and start another one. this also explains why the file is "overwritten" - because it is never actually in your downloads folder and is still on your downloads list.
If you think the file is legit then check if there is any metadata - height, width, colors - listed for it (is the details panel empty). If there is data there then you should be able to "forget all known sources" (
right-click > properties > actions). If the file does not have any metadata then there is a chance that it is spam(I have yet to see spam results with metadata). Make sure that you have a good
anti-spam filter and that it is properly installed.