substitution cs50 - handle repeating characters in key - c

Question

This code is for the CS50 Harvard course Pset 2 substitution.
one section of my program requires a check on the key to make sure characters are not repeated. I am failing this check as it reads ' :( Handles duplicate characters in Key - timed out while waiting for program to exit'
What needs to be fixed in my code to pass this final check?
#include <cs50.h>
#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, string argv[])
{
printf("\n");
//check if we have correct number of command line arguments
if (argc != 2)
{
printf("Usage: ./substitution key\n");
printf("(Please enter only 2 command line arguments)\n");
return 1; //error
}
//initialize global variables for use//
//length of key
int s = strlen(argv[1]);
//copy of key may be altered if it is not valid yet
string k = argv[1];
// if else to check if our key is 26 characters
if (s == 26)
{
for (int i = 0; i < 26; i++)
{
//checks each index in k to see if it is alpha or not, throws error if not
if (!isalpha(k[i]))
{
printf("Usage: ./substitution key\n");
printf("(Key must be alphabetical)\n");
return 1; //error
}
}
for (int i = 0; i < s; i++)
{
for (int j = i + 1; j < s; j++)
{
if (isupper(k[i]))
{
k[i] = tolower(k[i]);
}
if (k[i] == k[j])
{
printf("Usage: ./substitution key\n");
printf("(Key can not have repeating characters)\n");
return 1; //error
}
}
}
}
else
{
//if we dont have 26 characters
printf("Usage: ./subsitution key\n");
printf("Key must be 26 characters\n");
return 1;
}
//true key for our cipher
string key = k;
//ask user for plaintext
string plaintext = get_string("Plaintext: ");
int n = strlen(plaintext);
printf("ciphertext: ");
char *ciphertext = malloc(n);
for (int i = 0; i < n; i++)
{
//plaintext = Hello (H - 65 = 7 (8TH INDEX) IN OUR KEY)
if (isalpha(plaintext[i]))
{
if (isupper(plaintext[i]))
{
int j = plaintext[i] - 'A';
printf("%c", toupper(key[j]));
}
if (islower(plaintext[i]))
{
int q = plaintext[i] - 'a';
printf("%c", tolower(key[q]));
}
}
else if (isdigit(plaintext[i]))
{
printf("%c", plaintext[i]);
}
else
{
printf("%c", plaintext[i]);
}
}
printf("\n");
}

Answer 1

check50 tells you what argument it uses for the test. When you run your code with that argument, does it complain about duplicate characters in the key?
This program will not find a duplicate if the second occurance of a character is in upper case.