Don't know if this has been discussed previously..
Anyway..

After watching MD many, many, many times, I realized that the first time we see Betty at the LA airport, when she's walking side by side with Irene, we can see, just behind her and Irene, a couple of women dressed in the same way..they're probably flight attendants (that would be my first guess).
Then, when we see Betty and Irene out of the airport, we only see one of them.

Maybe it doesn't have any particular meaning, but this thing captured my attention since the first time I realized it.

I know for sure David Lynch is very meticulous about the staging of his scenes, so I guess that the choice of two people dressed in the same way in a single scene wasn't accidental.

100% agree that it is absolutely no coincidence that those twins are shown behind Betty and then we only see one of them leaving. There were some ideas kicked around a while ago back on RT about it and nobody seemed to quite pin it down.

It could be just another reference to the duality themes running through the film or that David seems to love twins in general, but the fact that we are shown only one leaving might appear to be significant in some sense at least.

A friend of the forum suggested to me a while back that there is a connection with a French film called L'Appartement. That connection is not a straightforward one to fathom and I will leave it to him to decide if he wants to share it up front, HOWEVER I would highly recommend that you check the film out regardless if you haven't seen it as it has influenced MD in other ways too.

I would love to talk to people about it as I looked at it very closely for quite some time. It has its own puzzles to solve.

For me, I think it ties into the moment where Betty disappears after Rita and she have gone into Aunt Ruth's bedroom with the blue box. I've always seen Betty and Rita as being a part of Diane. Betty is the more shallow, physical aspect and Rita is the soul. Betty is the box and Rita is the key. When they merge there is need for only one, just as when Rita opens the box she falls into it and wakes up as Diane. 2 becoming 1.

And Betty and Rita become one in a physical sense before Silencio. Both with the lovemaking and this shot inspired by Bergman's Persona.

Exactly! It's one of the major themes of the film.

And what about the blue van in front of the old couples limousine?