Normally, GCC does not warn about a second definition of a preprocessor macro with the exact same expansion:

    #define some_ordinary_macro 1
    #define some_ordinary_macro 1   /* no diagnostic */

However, GCC 9 and 10 *do* complain about this kind of redefinition if the name of the macro begins with __STDC_:

    #define __STDC_anything 1
    #define __STDC_anything 1     /* warning: "__STDC_anything" redefined */

In particular, it will complain about double definitions of the various __STDC_WANT_*__ macros.  The upcoming autoconf 2.70 will start defining many of those macros when the script uses AC_USE_SYSTEM_EXTENSIONS, and this broke code in APR's configure script that included the internal 'confdefs.h' (which accumulates the various #defines that will be emitted into config.h) a second time (autoconf test programs always include confdefs.h before doing anything else)

I think GCC should treat __STDC_ macros the same as ordinary macros - no warning for a second definition with the same expansion.  N.B. clang already behaves this way.

(If the second definition has a *different* expansion, GCC complains regardless of the macro's name, and that behavior should not be changed.)