Hi, I am writing a program which is to contain a list of library books. My problem is that the program is not too allow 2 ISBN numbers the same to be entered. I have used strcmp to check that the ISBN numbers are not the same, but I cannot get this to work, correctly. At the moment the program allows me to enter one ISBN then jumps to say ISBN already entered. Can anyone see where I am going wrong with my code. Thanks.

Code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/* Data Structure */
typedef struct
{
 char ISBN[20];     /* ISBN Number of book*/
 char author[20];   /* Author of book*/
 char title[30];    /* Title of book*/
 
} library_book;
/* global variables */
FILE * bookfile;      /* File of books*/
library_book books;   /* Array of books*/
int choice=0;         /*Variable to allow menu choice*/
/* Declare Functions */

void menu (void);         /*Function to run menu*/
void add_book (void);     /*Function to add a book*/
void list_books (void);   /*Function to list books*/
void search_ISBN (void);  /*Function to searc for an ISBN*/
void search_title (void); /*Function to search for a title*/

int main (int argc, char *argv[])

 
/******************************************************************************/
/*Function to add a book */
  void add_book (void)
  
  
  {             int found = 0; /* Variable to allow ISBN check*/
  
                system ("cls"); /* Clear the Screen */
       
                bookfile = fopen("BookFile.bin","ab"); /* Open File */
       
     if (bookfile == 0)
                {  printf ("An error occurred while opening the file.\a\n");
                  
                   }/* End of if statement*/
                  
     printf ("Please enter ISBN or q to quit.\n");
     scanf ("%s", &books.ISBN);
                     
     strcmp (books.ISBN,books.ISBN);
                                                     
     /* Do  the following until user wishes to quit or ISBN = ISBN*/       
             
     while ((tolower(books.ISBN[0]) != 113)&&(strcmp (books.ISBN,books.ISBN)==1))
              found =0;
                                                       
              { 
              printf ("Please enter author.\n");
              /* flush buffers and use gets to enable spaces */
              fflush(stdin);
              gets (books.author);
              /* Check string length is not too long*/
              while((strlen(books.author)<1)||
                    (strlen(books.author)>20))
                    
                    {
                    printf ("Author between 1 and 20 characters.\n");
                    fflush(stdin);
                    gets (books.author);
                    }/*End of error check*/
                            
              printf ("Please enter title of book.\n");
              /* flush buffers and use gets to enable spaces */               
              fflush(stdin); 
              gets (books.title);
              /* Check string length is not too long*/
              while((strlen(books.title)<1)||
              (strlen(books.title)>20))
              
                    {
                    printf ("Title between 1 and 20 characters.\n");
                    fflush(stdin);
                    gets (books.title);
                    }/*End of error check*/
                     
              fwrite (&books,sizeof(books),1,bookfile); 
              
              /*Error message for if ISBN is already of file*/               
              strcmp (books.ISBN,books.ISBN);
              while (strcmp (books.ISBN,books.ISBN)==0)
            
                    { 
                    printf ("Book is already on file, Enter another ISBN\n");
                    scanf ("%s", &books.ISBN);
                    found = 1;
                    }/* End of while loop to check ISBN*/        
                                                                                                 
               printf ("Please enter ISBN or q to quit.\n");
               scanf ("%s", &books.ISBN);
                                                                                 
              }/* End of while loop */
                                                                    
       fclose (bookfile);  /* Close the file*/
       
       }