input: Write a function to find duplicates in an array.
  assert find_duplicates([3, 5, 6, 7, 3, 5]) == [3, 5]
  assert find_duplicates([1, 2, 3, 4, 5]) == []
  assert find_duplicates([1, 1, 1, 1, 1]) == [1, 1, 1, 1]
output: def find_duplicates(arr):
      seen = set()
      duplicates = []
      for num in arr:
          if num in seen:
              duplicates.append(num)
          else:
              seen.add(num)
      return duplicates