Learn Turkish: Bavul havaalanında bir yerde duruyor. - The suitcase is sitting somewhere at the airport.

Bir yer literally means "a place", and it's used in Turkish as the indefinite pronoun "somewhere". Because the suitcase is located in that place, we have to add the locative case ending -de (in/at/on), making it bir yerde (in a place / somewhere).

Both havaalanında and bir yerde have the "at/in" ending (-da and -de). Is it normal to use two in a row?

Yes! In English we say "somewhere at the airport," but in Turkish, both the specific location (havaalanında) and the indefinite place (bir yerde) take the locative case. It translates quite literally to "At the airport, at a place."

Why is duruyor translated as "is sitting" here? Doesn't durmak mean "to stop"?

Durmak does mean "to stop", but it also means "to stand" or "to be positioned". When talking about inanimate objects like a suitcase (bavul) or a parked car, duruyor is the natural way to say it "is sitting" or "is resting" somewhere.

What is the extra -n- doing in havaalanında?