Video uploads already deny video URLs that have already been uploaded. I often don't bother checking to see if a specific video has already been uploaded because of this. I also take my time when I make tags.
When all that time making good tags is wasted because a video has already been uploaded, it sucks. One solution would be to hastily add a video, submit it, then edit it if it is successful. What I'm proposing instead would be a button to check if a video has already been submitted.