C++17: __has_include

C++17 added a macro that takes a header file name as an argument, and returns 1 if it was included already, and 0 if it was not. Here is an example:

#include <iostream>

#if __has_include(<iostream>) == 0
  #define INCLUDED_IOSTREAM_AGAIN true
  #include <iostream>
#else
  #define INCLUDED_IOSTREAM_AGAIN false
#endif

int main()
{
  if (INCLUDED_IOSTREAM_AGAIN)
  {
    std::cout << "Included  again." << std::endl;
  }
  else
  {
    std::cout << "Did not included  again." << std::endl;
  }
  return 0;
}
// Output: Did not included  again.

Reference: https://en.wikipedia.org/wiki/C%2B%2B17