ISOGRAM

An isogram is a word that has no repeating letters, consecutive or non-consecutive. Implement a function that determines whether a string that contains only letters is an isogram. Assume the empty string is an isogram. Ignore letter case.

is_isogram("Dermatoglyphics" ) == true
is_isogram("aba" ) == false
is_isogram("moOse" ) == false # -- ignore letter case

SOLUTION : 
IN PYTHON :

def is_isogram(string) :
    if string == '' :
        return(True)
    list = []
    list.extend(string.lower())

    for i in range(len(list)) :
        repeatation = list.count(list[i])
        global repeat_count
        repeat_count = 0
        if repeatation > 1 :
            repeat_count += 1
            return(False)
            break
    if repeat_count == 0  :
            return(True)

print(is_isogram('isogram'))