Find the first repeated character

Find the first repeated character in a given string.

def first_repeated_char(S):

    for index, c in enumerate(S):
        if S[:index+1].count(c) > 1:
            return c

    return "None"

print(first_repeated_char("abcdabcd"))    # a
print(first_repeated_char("abcd"))        # None

See also

https://www.w3resource.com/python-exercises/string/python-data-type-string-exercise-53.php