If there are multiple copies of the header file I'm not sure which one the code completion plugin will use (or even whether it consistently uses the same header). And mingw-w64 has different files like these:
\mingw32\lib\gcc\i686-w64-mingw32\4.8.1\include\stdint.h
\mingw32\i686-w64-mingw32\include\stdint.h
etc..
CC first try to locate all the search dirs when start a "batch parse"
See: const wxArrayString& NativeParser::GetGCCCompilerDirs(const wxString &cpp_compiler)
When parsing files, it see the #include "xxx.h", then try to search xxx.h file through all the search dirs, and use the first found file, see
wxString ParserBase::GetFullFileName(const wxString& src, const wxString& tgt, bool isGlobal)