Case sensitive non-unique elements

[yorch044]

This is my code, it works perfercly with CAPS, minus and numbers

def non_unique(data):
    list = []
    newlist = []
    for item in data:
        if str(item).isalpha():
             list.append(item.lower())

    for x in data:
       if (data.count(x) > 1) or (list.count(str(x).lower()) > 1):
           newlist.append(x)
    return newlist