Find the first repeated character with smallest first occurrence indexΒΆ

Find the first repeated character of a given string where the index of first occurrence is smallest.
def first_repeated_char_smallest_distance(S):
    temp = {}
    for c in S:
        if c in temp:
            return c;
        else:
            temp[c] = 0
    return 'None'

# test
print(first_repeated_char_smallest_distance("abcabc"))     # a
print(first_repeated_char_smallest_distance("abbcabc"))    # b
print(first_repeated_char_smallest_distance("abcbabc"))    # b
print(first_repeated_char_smallest_distance("abcxxy"))     # x
print(first_repeated_char_smallest_distance("abc"))        # None