Exercise 2.60.  We specified that a set would be represented as a list 
with no duplicates. Now suppose we allow duplicates. For instance, the 
set {1,2,3} could be represented as the list (2 3 2 1 3 2 2). Design 
procedures element-of-set?, adjoin-set, union-set, and intersection-set 
that operate on this representation. How does the efficiency of each 
compare with the corresponding procedure for the non-duplicate 
representation? Are there applications for which you would use this 
representation in preference to the non-duplicate one?

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Element-of-set? will be unchanged and is still O(n), where n is now the 
number of items in the representation, which might be greater than the 
cardinality of the set.

Adjoin-set can be implemented as cons and becomes O(1).

Union-set becomes a simple append operation which is O(n) in the size 
of the first list.

Intersection-set is unchanged except that, as for element-of-set?, it 
will become slower with more redundancy in the representations.

In an application where adding elements is very common, testing set 
membership is uncommon, or when the majority of elements added are 
distinct anyway, this representation may be preferred.