I have an include (nvidia GL/glext.h) which incorrectly defines a typedef twice.  However gcc doesn't not always report this is as a failure.

mig27 12:16:21$ cat test.c
#include "GL/gl.h"

mig27 12:15:23$ gcc -E  test.c > a.i
mig27 12:15:30$ gcc -c a.i
mig27 12:15:34$ echo $?
0
mig27 12:15:36$ gcc -E -P test.c > a.i
mig27 12:15:43$ gcc -c a.i
a.i:1505: error: redefinition of typedef ‘PFNGLGETUNIFORMUIVEXTPROC’
a.i:1494: error: previous declaration of ‘PFNGLGETUNIFORMUIVEXTPROC’ was here