why is my code not executing? and no clue where i'm going wrong? : Im fairly new to programming - my only experience is in schooling environments, as much as i have done is visual basic

Currently learning C - and i have no clue where i'm going wrong 

        #include <stdio.h>
    
     int main()
    {
    	char alphabet[20];
    	int i;
    
    	for (int 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;
    	}
    
    }

The program that i am asked to make for class - im required to create a 1D array, add validation for alphabetical letters and duplicate letters as well as creating a function for sorting the letters and specifying the number of times each letter was put in

As much as ive been able to attempt coding is
 - Create a 1D array to read 20 alphabetical letters
 - Add validation for duplicate letters and printf 'Duplicate Letters'
but everytime i try, the program terminates at 'Enter in a letter:' or it wont execute

Where did i go wrong?

For background - I work mainly on windows 7 as thats what the school has - using MinGW as my compiler, but for working at home i use MacOS using Terminal as the compiler
