Find unique characters and count non unique characters
A simple and pythonic way to accomplish this is using collections.Counter. It's an easy way to count elements in an iterable. Have a look:

from collections import Counter

def unique_letters(string: str) -&gt; str:
    """
    Finds all the unique/non unique letters in the string
    """
    letters = Counter(string)

    unique = [key for key in letters if letters[key] == 1]
    non_unique = {key: letters[key] for key in letters if letters[key] &gt; 1}
    return f"Unique: {unique} | Not Unique: {non_unique}"


This also takes advantage of f-strings, allowing you to directly implement variables into your strings. It's a personal preference, but to me it looks a lot nicer than using .format().