Geoffr, I've read that too, but I don't think the two approaches are mutually exclusive. I think Cooper is much more aware of his new-found dark side than Leland was. I tend to like the theory that the boy in FWWM is a representation of Leland while the jumping man in Philip's vision is a representation of Dale. The boy's mask significantly has no eye holes while the jumping man's mask does have them, suggesting that maybe Cooper can see his dark self while Leland can not.
I believe that's why Cooper smashes the mirror. He can - for a fleeting moment - see what he's become. Leland could never see that completely until just before he died. Cooper, on the other hand, wants to destroy the reflection of his bad side even if it's useless. To me, if the mirror-smasher is the real Dale or the Doppelganger is a red herring. They're both aspects of Dale, and he probably needs to go back to the Lodge for a second shot, maybe with Briggs or something to restore the integrity of his soul (if it's possible at all).
Even if Lynch feels that they're completely different entities, there is no real answer in either episode 29 or in FWWM, no matter what he intended at the time. Annie's statement isn't clear-cut at all, and the one clear indication is in a cut or unfilmed part of the script. I prefer to think he kept it deliberately ambiguous. And I think he was right to do so. If that part had stayed in the film, I would have complained and we wouldn't have had this nice discussion about the nice ambiguity of Twin Peaks.
"What credit card do you want to put that on?"
"Caash, prease."