def nth_replace(str,search,repl,index):
    split = str.split(search,index+1)
    if len(split)<=index+1:
        return str
    return search.join(split[:-1])+repl+split[-1]
str1 = "caars caars caars"
str2 = nth_replace(str1, 'aa', 'a', 1)
print(str2)

caars cars caars

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