Find duplicates under given constraints

A sorted array contains 6 different numbers, only 1 number is repeated five times. So there are total 10 numbers in array. Find the duplicate numbers using two comparisons only. 

Examples : 

Input: arr[] = {1, 1, 1, 1, 1, 5, 7, 10, 20, 30}
Output: 1

Input: arr[] = {1, 2, 3, 3, 3, 3, 3, 5, 9, 10}
Output: 3

Asked in Yahoo

An important observation is, arr[4] or arr[5] is an occurrence of repeated element for sure. Below is the implementation based on this observation. 

Implementation:

1

Time Complexity: O(1)

Auxiliary Space: O(1)

Exercise: Extend the above problem for an array with n different elements, size of array is 2*(n-1) and one element repeats (n-1) times.