Ironically, I'm going to dupe my comment from Zeph's post
http://www.videosift.com/talk/The-sift-needs-concrete-rules-on-dupes
because this isn't so much about the rules for what is and isn't a dupe as it is about how to notify people a submission might be a dupe.

I think a *dupe invocation should be added and available to standard members. This would add it to channel "dupes" but not do anything else and leave it in the queue with the video still available to be voted on.

The reason I say this is because obviously a lot of people view and vote on videos without ever going to the specific page for it where the comments are (and where a member may be pointing out that the video may be a dupe.

The channels the video are associated with are however visible without ever going to the video's page so with *dupe at least someone scrolling through the unsifted page (or similar) would see that channel designation and then they could discuss, discard or at least be aware it might be a dupe before voting.

This creates a sort of dupe notification system so that the members with the ability and inclination can deal with these dupes, remove the *dupe tag if it's not a dupe .. whatever.

I say standard members should be able to do this because higher tier members already have tools to deal with the situation.