input: Write a function to find duplicates in a list with linear time complexity.
  assert find_duplicates_linear([1, 2, 3, 4, 5, 1, 2]) == [1, 2]
  assert find_duplicates_linear([10, 20, 30, 40, 50, 10, 20, 30]) == [10, 20, 30]
  assert find_duplicates_linear([5, 5, 5, 5, 5]) == [5, 5, 5, 5]
output: def find_duplicates_linear(lst):
      seen = set()
      duplicates = []
      for num in lst:
          if num in seen:
              duplicates.append(num)
          else:
              seen.add(num)
      return duplicates