* FIX: Video thumbnails can have duplicates
It's possible that a duplicate video or even a very similar video could
generate the same video thumbnail. Because video thumbnails are mapped
to their corresponding video by using the video sha1 in the thumbnail
filename we need to allow for duplicate thumbnails otherwise even when a
thumbnail has been generated for a topic it will not be mapped
correctly.
This will also allow you to re-upload a video on the same topic to
regenerate the thumbnail.
* fix typo