C1009: Invalid redeclaration

[ERROR]

Classes, structures and unions may be declared only once. Function redeclarations must have the same parameters and return values as the original declaration. In C++, data objects cannot be redeclared (except with the extern specifier).

  struct A {
    int i;
  };

  struct A { // error
    int i;
  };  

Avoid redeclaration, e.g. guarding include files with #ifndef.