In Firefox, on a page with a video, I clicked the VDH arrow to start a quick download. Smart name grabs the title from the title bar, naming the file "Storms. - Index.mp4". Go to another video on the same site, it has the same name, so it downloads "Storms. - Index.mp4" again. Go to the download list in Firefox, see the two videos, one is (x)MBs large, the other is (y)MBs large. Open the second video from the list, it plays just fine. Click the first video on the list, the size changes from (y)MBs large to (x)MBs large as it reads the file size from disk. Open the video, it's the same as the second video. Go to the download folder, there is only 1 video.
The video appeared to download normally, but there were a bunch of entries in VDH in a section labeled "logs" which said "Download Cancelled". I was also downloading other things so I don't know if that was directly related or not, as there was no other information in the log.
I tested this with downloads via Firefox itself, without VDH, and this problem does not occur.