Find duplicate element in array in time O(n)

Question

I have been asked this question in a job interview and I have been wondering about the right answer.

You have an array of numbers from 0 to n-1, one o

Answer 1

int[] a = {5, 6, 8, 9, 3, 4, 2, 9 };
int[] b = {5, 6, 8, 9, 3, 6, 1, 9 };

 for (int i = 0; i < a.Length; i++)
  {
     if (a[i] != b[i])
      {
       Console.Write("Original Array manipulated at position {0}  + "\t\n"  
                             + "and the element is {1} replaced by {2} ", i, 
                             a[i],b[i] + "\t\n" );
       break;               
      }      
  }
   Console.Read();

   ///use break if want to check only one manipulation in original array.
   ///If want to check more then one manipulation in original array, remove break