best way to add item to list only once

[Phaze90]

Hi,

what is the more effective way of the both below?
Or is there a third variant?

def add_only_once_to_list(in_obj, in_list,):
    if in_obj not in in_list:
        in_list.append(in_obj)

Or with a local set:

def add_only_once_to_list(in_obj, in_list,):
    temp_set = set(in_list)
    temp_set.add(in_obj)
    in_list = list(temp_set)