How is a duplicate video defined? Are clips from longer sifts okay? Are longer videos that contain shorter, previous sifts okay?

I'm inclined to say that both can be useful, but the second more than the first. I'd rather see a longer version of a video posted, than a shorter section from an existing video. With that in mind the FAQ would have to be changed.

http://www.videosift.com/faq-en.php#posting_guidelines

A duplicate video is one which contains content already on VideoSift in a published, queued, personal queued, or dead video submission. Minor changes in content, like a few additional insignificant seconds of video or alternate background music, will still be considered dupes.

What do you think?