Given a list of marks, marks = [20, 23, 22, 23, 20, 21, 23]. List all the duplicates 

If a = [1, 1, 2, 3, 3, 5, 5, 8]. What is set(a)
        set([1, 1, 2, 3, 3, 5, 5, 8])
        set([1, 2, 3, 5, 8])
        set([1, 2, 3, 3, 5, 5])
        Error

odd = set([1, 3, 5, 7, 9]) and squares = set([1, 4, 9, 16]).
 How do you find the symmetric difference of these two sets?

a is a set. how do you check if a variable b exists in a?