That's what i was saying before. Any youtube user (us) can report a video with 4 simple clicks.
But there isn't a valid option to report a duplicate video from another user.
The only options that are close are "Infringes my rights" then.. "Infringes my copyright" or "other legal claim" but they only apply to dave and not us.
Going through 300 vids and clicking 4 times doesn't sound to hard.
I'm sure there are people on here that would do it for dave but he'd need to trust them with his account access for a short time.
You could probably streamline it by opening 100 tabs at a time, one per video, then keeping the mouse in the same location over the report button and use keyboard shortcuts to change tabs, click, change tab, click etc..
Then start from tab1 again with the mouse of the next thing to click.
Probably only take 30min or so.
Note: i've not actually reported anything so i don't know if it asks any more questions once you select the reason and hit "Submit".