In the book Introduction to the Design & Analysis of Algorithms, the following solution is proposed to the element uniqueness problem:

ALGORITHM UniqueElements(A[0 .. n-1])
// Determines whether all the elements in a given array are distinct
// Input: An array A[0 .. n-1]
// Output: Returns "true" if all the elements in A are distinct
//         and false otherwise.
for i := 0 to n - 2 do
   for j := i + 1 to n - 1 do
      if A[i] = A[j] return false
return true

How can I compute the average cost (i.e. number of comparisons for a given n) for this algorithm? What is a reasonable assumption about the input?