Duplicate cIncludes detection

[Swoogan]

This isn't a bug per se. Take the scenario:

a.zig

pub usingnamespace @cImport({
    @cInclude("gtk/gtk.h");
});

pub usingnamespace @import("b.zig");

b.zig

pub usingnamespace @cImport({
    @cInclude("gtk/gtk.h");
});

Building this will result in over 2000 errors, mostly pertaining to redefinitions. After doing this I gained a better understanding of what I was actually asking the compiler to do and it's clear that what I was asking it to do is stupid, but I feel like this might be an opportunity for the compiler to fail early. One error message stating "Attempt to re-include header 'gtk/gtk.h' in the same scope" would be better than 2000 symptoms of the root problem.

[daurnimator]

It would make more sense to be duplicate @cImport detection rather than @cInclude duplication: sometimes you have libraries where you include the same header multiple times with different defines set.

That said, I don't know if this should be an error. Two unrelated zig libraries might @cimport the same C library. There is no reason for that to be an error.

[pixelherodev]

I think making it so that the redefinitions don't occur is better than marking what should in some situations succeed as an error.

[andrewrk]

#20630