foo.cpp: In function 'double fabs(double)':
foo.cpp:30:8: error: redefinition of 'double fabs(double)'
 double fabs(double x){
        ^~~~
In file included from C:/mingw32/lib/gcc/i686-w64-mingw32/6.3.0/include/c++/cmath:45:0,
                 from foo.cpp:5:
C:/mingw32/i686-w64-mingw32/include/math.h:230:31: note: 'double fabs(double)' previously defined here
   __CRT_INLINE double __cdecl fabs (double x)
                               ^~~~