索引未发现的字母出现2次

def find_missing_letter(s):
    char_count = {}
    for i, char in enumerate(s):
        if char in char_count:
            char_count[char][0] += 1
        else:
            char_count[char] = [1, i]
    
    for char, count_index in char_count.items():
        if count_index[0] == 2 and count_index[1] == -1:
            return char
    
    return None

# 示例用法
s = "abbcdefg"
result = find_missing_letter(s)
print("索引未发现的字母出现2次:", result)