input: Write a function to remove all the duplicates from a linked list in Python.
  assert remove_duplicates([7, 14, 28, 28, 28, 35, 49])==[7, 14, 28, 35, 49]
  assert remove_duplicates([1, 1, 1, 1, 1])==[1]
  assert remove_duplicates([5, 10, 15, 20, 25])==[5, 10, 15, 20, 25]
  assert remove_duplicates([2, 2, 4, 4, 4, 6, 6])==[2, 4, 6]
  assert remove_duplicates([])==[]
output: def remove_duplicates(lst):
      seen = set()
      result = []
      for item in lst:
          if item not in seen:
              seen.add(item)
              result.append(item)
      return result