Wasn't sure what they meant by tulpa but it's tying in Tibet to the new series
"Tulpa (sprul pa, སྤྲུལ་པ) is a Tibetan word meaning ’emanation’, ‘apparition’ or ‘magical illusion’, and the practices of contemporary tulpamancers are supposedly modeled on the esoteric procedures of Tibetan mystics, who we are often told, have been producing sentient mental-entities through meditation for centuries."
So potential explanation on how Dougie / Fake Diane came about from Bob Cooper, this may also be why Laura Palmer showed up in ep8 in that gold seed that fell to earth. I'm still not entirely sure who the guy was in the Twin Peaks jail that was drooling and parroting everyone, seems like another tulpa going haywire.
This might also explain Sarah Palmer as well, perhaps when Leland was possessed by Bob he replaced her back in the original series.