将所有重复项替换为na

def replace_duplicates(string):
    unique_chars = []
    result = ""
    for char in string:
        if char not in unique_chars:
            unique_chars.append(char)
            result += char
        else:
            result += "na"
    return result

input_string = "abbcdeffggh"
output_string = replace_duplicates(input_string)
print(output_string)