#include <stdio.h>

 int main()
{
    char alphabet[20];
    int i;

    for (i = 0; i <= 20; i++)
    {
    printf("Enter in a letter:\n");
    scanf("%s", alphabet[i]);
        if (alphabet[i] == alphabet[i+1])
        {
            printf("Duplicate Letters");
        };
    return 0;
    }

}

Enter in a letter:
Duplicate Letters