Why include <stdlib.h> twice?

I'm reading source code and I found:

#include <stdio.h>             
#include <stdlib.h>            
#include <ctype.h>             

#ifdef HAVE_CONFIG_H           
#include "config.h"            
#endif                         

#include <math.h>              
#include <ctype.h>             
#include <stdlib.h> 

Why include <stdlib.h> and <ctype.h> twice？ Btw, I'm C beginner.