Dev - C++ - Leer una frase y determinar que caracteres se encuentran repetidos

#include <iostream>

#include <cctype>

int main()

{

    int ch;

    char c;

    int letras[27] = {0};

    std::cout << "\nFrase: ";

    while ((ch = getchar()) != '\n')

    {

        c = ch;

        c = toupper(c);

        if (isalpha(c))

            letras[int(c) - 65]++;

    }

    std::cout <<  std::endl;

    for (int i = 0; i < 27; i++)

        if (letras[i] > 1)

            std::cout << char(i + 65) << ": " << letras[i] << std::endl;

    return 0;

}