Exercism: determine if a word or phrase is an isogram
You can iterate once over text and short-circuit right away if any character is not unique.
import string


def is_isogram(text):
    seen = set()
    for char in text:
        if (lower := char.lower()) in string.ascii_lowercase:
            if lower in seen:
                return False
            else: 
                seen.add(lower)
    return True