Given an unsorted array, find duplicate elements in constant time, without using inbuilt functions of python.

To find duplicate elements in an unsorted array in constant time, you can modify the array in-place. Iterate through the array. For each element x, go to the index abs(x). If the element at that index is already negative, it means you've seen x before, so it's a duplicate. Otherwise, make the element at index abs(x) negative to mark it as seen. This approach requires the array elements to be within the bounds of the array's indices and assumes no negative numbers initially, or that you can handle them appropriately (e.g., by using a separate mapping or adjusting indices).