I was also also perplexed by the concept of the doppelgänger in The Return. It led me to reflect on the last episode of Season 2 where we see a doppelgänger for BOB and
one for Laura. And we see Diane's doppelgänger in Part 18 when they arrive at the motel, and I've always thought initially that Dopplecoop murdered Diane when he came out of Glastonbury Grove.
That's how I've always understood it, and how Cooper was able to appear to Laura in her dream: because of the timelessness of the lodge, once you're there you've always been there.
Didn't Ronnet say to Coop in the mauve room: "When you get there, you will already be there.
" Also notice the numbers over the outlets in Part 3 correspond to Parts in which the outlets are used to travel between worlds.
Furthermore, I think it's important to note that Dopplecoop entered Jack Rabbit's Palace at 2:53, where he was then transported to the TP Sherrif's station. Much time elapses from the moment he is teleported to the defeat of BOB, and yet, when Coop looks at the clock on the wall it appears to be stuck at 2:53. Either the clock had (symbolically) stopped working at 2:53, or perhaps that entire ordeal transpired in a timeline that was suspended by The Giant/Fireman.
The timeline confusion continues at the beginning of Part 18. Considering that Coop places the ring on Dopplecoop at the end of the battle in Part 17, it seems strikingly important to note that we do not actually see Dopplecoop return to The Waiting Room until the beginning of Part 18, long after Coop goes back in time to FWWM to save Laura.
But the biggest WTF in all that is the fact that the body of Dopplecoop transmutes back into a gold pearl in Part 18. So then doesn't that mean that what went back to the Lodge/Waiting Room was actually a Tulpa and not Dopplecoop? Dougie turned into a gold pearl in Part 3, and he was a Tulpa. White haired Diane's Tulpa turns into a gold pearl... So then WTF was Dopplecoop's flaming corpse doing turning into a gold pearl if it was not manufactured for a purpose? Remember when Coop finally regains consciousness in the hospital? He peeks into the Lodge for a moment to talk to Mike and says rather desperately: "I need another one!
" He then proceeds to hand Mike (through the curtain no less) a chunk of his hair. So is he asking Mike to create another Tulpa of himself and is that what we see in the beginning of Part 18?
For a moment there I began to wonder if the scene with the bomb was showing entities crossing over from one reality or timeline to another.
I suspect you are at least partially right in your observation Mr. Strawberry, because inside the atomic bomb explosion there are several moments depicted in color that are weaving in and out of the black & white footage. And we already know that what we are shown in b&w throughout The Return is intended to be seen as past and that color represents the closest thing to present in terms of timelines. So the Trinity test obviously had something to do with the physics of timelines in Twin Peaks.