Algorithm to find a duplicate entry in constant space and O(n) time

Question

Given an array of N integer such that only one integer is repeated. Find the repeated integer in O(n) time and constant space. There is no range for the value of integers or

Answer 1

Since extra space is not allowed this can't be done without comparison.The concept of lower bound on the time complexity of comparison sort can be applied here to prove that the problem in its original form can't be solved in O(n) in the worst case.