It happens occasionally. An example would be that there are a couple of libraries that do "generic collections" in C using the preprocessor (instead of using templates). So, you could imagine something like this:

    #define VEC_TYPE double
    #include <c-vector.h>
    #undef VEC_TYPE

and that would provide you a type like "vec_double", a vector specialized for the double type. You'd want to import that once for each vector type you wanted.

It's not the most common use-case (and you can argue if it's a good idea at all), but it is at least a reasonable reason why you'd want a header file without #pragma once.

some headers are meant to be included multiple times with different definitions (although templates are usually a better way to do this).