Let's assume that Frost's retweet is a tacit confirmation, and see where this goes. Let's start with the pessimistic, 'nothing-means-anything, Lynch-is-a-troll' options that I see circulating again, and then gradually consider more optimistic / charitable possibilities.
On a pragmatic level (and not meaning to be in any way rude to the actor) Blevins has an nice amount of forehead available for staging a head wound. And the fact this particular role is uncredited could be taken as a sign that it may not have been intended as significant who was playing the part; maybe Lynch just grabbed one of the most appropriate seeming cast members to fill a very minor, non-speaking role that hadn't been cast or whose intended player didn't make it for some reason.
However, if this was the case it must have been obvious that fans would spot the doubling up. Which leads us to the conclusion that either i) Lynch considered this, and thought the little bit of added mystery accentuated the story or ii) it was always intended from the beginning.
If it was part of the long-term picture to have Blevins reused here, then once again we are faced with two possibilities: either i) this character is Tommy (explaining why he is not credited as a separate character, but not explaining why Tommy is not credited for this episode) or ii) this is somebody else, in the sense that Carrie is somebody else to Laura (but this still does not account for the lack of mention in the credits, which usually carry these doubles as 'Carrie / Laura' or 'Richard / Cooper').
Assuming it wasn't just a roll of the dice decision as some folks are claiming on Reddit (which is also perfectly acceptable to me artistically by the way), our confusion at this re-use of an actor seems to stem from the fact that we don't know which of these above conclusions to arrive at. We don't know who Blevins is playing here, or even -- and this is the kicker -- if the apparent doubling is supposed to carry any informational content or is just an aesthetic, formal, choice made purely for the way it deepens the already deeply uncanny affective quality of this particular mise en scène.
We can't ask Lynch, he'll no doubt play death of the author on us. Frost is a possibility. Maybe someone in his social network could tweet out a few theories and see if any get a like or a retweet. Does Blevins have online presence? We could ask who he thinks he was playing, or what he was told.
My (provisional) thinking is this: Carrie's place is a setting that been carefully arranged to suggest certain things about her lifestyle. There are knocked over plant pots and tin cans in the front yard, which is uncultivated and barren; a car wheel leans against the outside wall, next to a dusty, unused garden chair; the paint on the doorframe is flaking away; the phone starts ringing urgently but Carrie will not take the call; the interior is drab and largely colourless -- the walls look to be bare plaster painted in a very pale olive, and look in places to need some patching up; the pictures that are hanging are folksy, generic and unmatched,as if some have been there for a long time and perhaps belonged to former, older, residents, while some (the three-panelled calligraphy display in the hallway area) are more modern additions; loo rolls are stacked up in the living area next to a vacuum cleaner and a trash can; the hearth / fireplace area is littered with what seems to be decorator's equipment. On a table, next to what may be either a giant coin bottle or an empty water cooler tank, beneath a very generic watercolour landscape and an equally generic tablelamp -- also extremely neutral in colour and unlit -- stands an empty portrait frame. Yes, it's blank. You don't arrange a setting this carefully, and have a lead character look around it, tracking his POV, unless you want it to mean something, be it to create an atmosphere or to do some narrative work. We've seen details like the white horse ornament, we've seen the telegraph / utility pole, but we are also shown this hulking cadaver of a man previously seen as a loan shark back in Dougie's Vegas. You don't make a backdrop this characterless and then drop significant details like the horse, the pole, the blank portrait frame, and a murder victim in it unless they are carrying some weight and doing some work for you. What is that work?
Well, this is Carrie Paige's place. She looks like Laura Palmer, and seems to somehow get stirred up by the mention of Sarah Palmer. She's played by the same actor as Laura. The white horse from Sarah's vision is on her mantelpiece. A portrait frame by the fireside -- not the same kind as the one that Sarah will later attempt to smash up, but still a significant detail -- lacks any memorable portrait photograph. The same telegraph pole we have seen many, many times now stands outside the apartment. The common theme here, running through all these elements of the mise en scène, is a largely visual collection that refers us back to the world in which Laura lived and died. We feel that this world is 'wrong' because while it is clearly different there is just enough familiarity in it to make it unheimlich. This is the work that all these items are doing, the load they are bearing.
À propos, it is not unreasonable to assume that the dead body with the gunshot wound also will also contribute to this air of disquieting familiarity, while also serving in its own right as a disturbing piece of set. This is especially the case if we recognise the actor, or find him 'eerily like' someone we have already seen on the other side. A great deal of the disquiet around this body, however, is generated by the fact that he isn't spoken about, and only really raises a passing, if shocked, reaction from the visiting Richard/Cooper. Otherwise he's very much treated as furniture, as set dressing. This works to further defamiliarise Richard, who is by now obviously no longer the Cooper we knew from before.
So, all in all the choice of this actor feeds into the thematic stream of this sequence: the play of the familiar and unfamiliar that is necessary for the generation of the uncanny affective qualities. If this was the result of a dice throw, it was a very, very, lucky throw.
What it means on a narrative, in-world level, I'll leave to fan theory.
I don't have one yet.
As a matter of fact, 'Chalfont' was the name of the people that rented this space before. Two Chalfonts. Weird, huh?