Why am I getting same characters with any key I type? - cs50

Question

Why am I getting this return 4 repeated characters for Pset2 Substition CS50
Everything seems to work fine except for the checking for repeated characters in the command line argument. Rest of the code works fine when I remove this return for the checking repeated characters. Everything I type gives me this error
#include <cs50.h>
#include <string.h>
#include <ctype.h>
#include <stdio.h>
int main(int argc, string argv[])
{
if (argc == 2)
{
//When there are 26 characters in string
if (strlen(argv[1]) == 26)
{
string check = argv[1];
//Checking each characters in string(array) where [i] is the array number
for (int i = 0, n = strlen(argv[1]); i < n; i++)
{
//If the characters in the string is alphabet or not
if (!isalpha(argv[1][i]))
{
printf("Key must only contain Alphabetic Characters\n");
return 3;
}
//Check for repeated alphabet
for (int l = 0; l < n; l++)
{
if (argv[1][i] == check[l])
{
printf("Key must not contain repeated Characters\n");
return 4;
}
}
}
//Get Plaintext
string plaintext = get_string("Plaintext: ");
//Going through each characters in string
for (int j = 0, n = strlen(plaintext); j < n; j++)
{
//For LOWERCASE CHARACGERS
if (plaintext[j] >= 'a' && plaintext[j] <= 'z')
{
plaintext[j] = argv[1][plaintext[j] - 97];
plaintext[j] = tolower(plaintext[j]);
}
//For UPPERCASE CHARACTERS
else if (plaintext[j] >= 'A' && plaintext[j] <= 'Z')
{
plaintext[j] = argv[1][plaintext[j] - 65];
plaintext[j] = toupper(plaintext[j]);
}
}
printf("Ciphertext: %s\n",plaintext);
}
//If Characters are not 26 in string
else
{
printf("Key must contain 26 Characters\n");
return 2;
}
}
//When Location is other than argv[1]
else
{
printf("./substitution KEY\n");
return 1;
}
}

Answer 1

When i is 0 and l is 0 what is being compared here if (argv[1][i] == check[l])? Actually whenever i and l are equal it is comparing a character to itself. Program does not need to compare any characters it has already processed. The compare should start at the current char + 1.