Write a function that will return the count of distinct case-insensitive alphabetic characters and numeric digits that occur more than once in the input string.

duplicate_count.py

def duplicate_count(text: str) -> int:
    """Write a function that will return the count of distinct case-insensitive
    alphabetic characters and numeric digits that occur more than once in the
    input string.
    
    The input string can be assumed to contain only alphabets
    (both uppercase and lowercase) and numeric digits.

    "abcde"            -> 0 # no characters repeat more than once
    "aabbcde"          -> 2 # 'a' and 'b'
    "aabBcde"          -> 2 # 'a' occurs twice and 'b' twice (`b` and `B`)
    "indivisibility"   -> 1 # 'i' occurs six times
    "Indivisibilities" -> 2 # 'i' occurs seven times and 's' occurs twice
    "aA11"             -> 2 # 'a' and '1'
    "ABBA"             -> 2 # 'A' and 'B' each occur twice
    """

    if not text: return 0
    
    text = text.lower()
    return len(set([char for char in text if text.count(char) > 1]))