How would I write a Python function to satisfy this doctest?

I have a test on Friday morning and can't figure out how to write this code. Assistance would be greatly appreciated!!

We have been using for & while loops, string slices, find() methods, etc.

Here is the question:

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'

"""