Three entrances at the airport

[Rachel Adams]

I don't understand why you insist on "specific". It's not about that. Forget about the airport for now. Which are you more likely to say: "I'm in a cafe downtown" or "I'm at a cafe downtown"? If you'd read Piscean's post above, you would have seen that the keyword there is "restricted".

I used "specific" instead of "restricted", in my post, but I was expressing the same idea, I guess. I would use "in" in the sentence you are asking about because it's a restricted location.
But if I am mentioning two places what should I use?
"There is a café in the airport." "In" if I am talking about a restricted place and "at" if I am talking about"airport" in a general sense. Then I should remove the second "in" before "airport" and use "at".

If there is only one restricted place in the second sentence then I should use "in" before "café" and "at" before "airport".

2. "I am in a café at the airport". The "restricted" location is "café" not the airport in the second sentence.

[teechar]

2. "I am in a café at the airport".

That's okay, but note that by "restricted", we're talking confined/has walls rather than specific/particular.

[Rachel Adams]

That's okay, but note that by "restricted", we're talking confined/has walls rather than specific/particular.

But regarding the "in/at" difference in the sentence below "in" would mean "restricted" as you explained but "at" isn't wrong either, is it? If I am talking about "the airport" in a general sense.
"There is a café in the airport"

[teechar]

Yes, that's right.

[Rachel Adams]

Yes, that's right.

One more question, if you please. "I am in a café in the airport" if I use "in" I would be again describing "restricted location." I just wasn't sure if using it twice is correct.

[teechar]

I just wasn't sure if using it twice is correct.

You're right. Using it twice in such close proximity is not ideal. That's why "at" would be better instead of the second "in".

[Rachel Adams]

You're right. Using it twice in such close proximity is not ideal. That's why "at" would be better instead of the second "in".

In "Essential Grammar in Use" by Raymond Murphy, there is a picture of an airport with people and it says "There are many people at the airport" it doesn't give "in the airport" as another possibile answer as it usually does. Is it just using it in a general sense?

[teechar]

In "Essential Grammar in Use" by Raymond Murphy, there is a picture of an airport with people and it says "There are many people at the airport" it doesn't give "in the airport" as another possible answer as it usually does. Is it just using it in a general sense?

An airport has terminal buildings, cargo areas, car parking facilities, runways, drop-off areas, etc. "At" covers them all.

[emsr2d2]

One more question, if you please. "I am in a café in the airport" if I use "in" I would be again describing "restricted location." I just wasn't sure if using it twice is correct.

I consider all of these equally acceptable:

I am at a café in the airport.
I am at a café at the airport.
I am in a café in the airport.
I am in a café at the airport.

[Rachel Adams]

I consider all of these equally acceptable:

I am at a café in the airport.
I am at a café at the airport.
I am in a café in the airport.
I am in a café at the airport.

In Michael Swan's "Practical English Usage" I read: "We very often use "at" before the name of a building, when we are thinking not of the itself but of the activity that happens there.

"I first heard her sing at the Usher Hall in Edinburgh."

The book gives another example and says: "It was warm and comfortable in the club." (A place to spend time). I don't understand its comment. I think it's more helpful to say as it was explained here that "in" is a restricted area. I may be wrong of course.
So "at" is used when speaking about a place generally and also when speaking about the purpose being there.