The problem
We need to avoid accidental double declarations. This may occur if we create a function inside a code that references the header file to a source code with the same function.
Solution
put #pragma once at the start of each header file.
This is what #pragma once does:
#ifndef SOME_UNIQUE_NAME_HERE
#define SOME_UNIQUE_NAME_HERE
// your declarations (and certain types of definitions) here
#endif