Given a string, remove any characters that are unique from the string.
Example: 
input: "abccdefee"
output: "cceee"
def only_duplicates(string):
    return "".join([x for x in string if string.count(x) > 1])
def only_duplicates(string):
    return "".join([x for x in string if string.count(x) > 1])