Reduplication of nouns have meanings other than pluralization, such as labeling a thing that has a similar concept as that of the root word. In this example, "Langit" means sky. But reduplicating it means a its-a-sky-thingy, which in this case, it's a ceiling.
There are other examples similar to "langit-langit", like:
- rumah -> house
- rumah-rumahan -> playhouse
- gula -> sugar
- gula-gula -> candy, same as "permen"
- tiba -> arrive, same as "datang"
- tiba-tiba -> suddenly