From reddit thread here. 

"Doppler made the original Dougie tulpah, Cooper makes the new Dougie tulpah. All season long we have commented that Doppler's movement and speech seems particularly strange at times, while the P18 'Richard' Cooper is clearly not the quintessential Cooper either.

One small supposition explains a lot; when you create a tulpah, you lose that part of yourself. Cooper gave up his childhood innocence and intuition, the qualities most obviously embodied by Dougie, and this explains Coop's lack of certainty throughout P18. Meanwhile, original Dougie gambled and got prostitutes: Doppler gave up pleasure (or a better holding word, maybe satisfaction). After all, Doppler is never satisfied in The Return, despite his 'want'. This also explains his difference from the highly eccentric Doppler we saw at the end of season 2.

This explains the weirdness of both Doppler and Cooper, and seems very appropriate. Am I missing something? Also, could there be implications for the Diane tulpah?

edit: To continue this idea slightly, if Cooper lost his 'childhood intuition' when he created the second Dougie, then the Cooper we see drive back into Twin Peaks in P17 is 'the one and only' because at that point in time the 'new' Dougie hasn't been created yet. This is the complete Coop, for now, and it's why he finally acts like the Coop from the original run. For the record, we don't see the new Dougie creation scene until P18, after our Cooper goes back 'in'. At this point, Cooper comes back out to see Diane, and essentially begins this theory by acting like he lost his intuition and/or childhood innocence."

While I'm not sure I agree with the thought that he lost his "childish intuition" (joy maybe, but not intution per se) I actually think the poster is on to something here when he/she discusses the loss of a part of yourself whenever a tulpa is made. . . like slicing off a piece of your soul to embue that thing with life.  . .