def remove_duplicates(strng):
    """
    Returns a string which is the same as the argument except only the
    first occurrence of each letter is present.  Upper and lower case
    letters are treated as different.  Only duplicate letters are removed,
    other characters such as spaces or numbers are not changed. 

    >>> remove_duplicates('apple')
    'aple'
    >>> remove_duplicates('Mississippi')
    'Misp'
    >>> remove_duplicates('The quick brown fox jumps over the lazy dog')
    'The quick brown fx jmps v t lazy dg'
    >>> remove_duplicates('121 balloons 2 u')
    '121 balons 2 u'
    """
    s = strng.split()
    return strng.replace(s[0],"")

Writing a function to get rid of duplicate letters but so far have been playing around for an hour and can't get anything. Help would be appreciated, thanks.