You need to append to the first list those elements of the second list that aren’t in the first – sets are the easiest way of determining which elements they are, like this:
first_list = [1, 2, 2, 5]
second_list = [2, 5, 7, 9]
in_first = set(first_list)
in_second = set(second_list)
in_second_but_not_in_first = in_second - in_first
result = first_list + list(in_second_but_not_in_first)
print(result) # Prints [1, 2, 2, 5, 9, 7]
Or if you prefer one-liners 😎
print(first_list + list(set(second_list) - set(first_list)))