How to return unique words from the text file using Python

Question

How do I return all the unique words from a text file using Python? For example:

I am not a robot

I am a human

Should return:

Answer 1

def unique_file(input_filename, output_filename):
    input_file = open(input_filename, 'r')
    file_contents = input_file.read()
    input_file.close()
    duplicates = []
    word_list = file_contents.split()
    file = open(output_filename, 'w')
    for word in word_list:
        if word not in duplicates:
            duplicates.append(word)
            file.write(str(word) + "\n")
    file.close()

This code loops over every word, and if it is not in a list duplicates, it appends the word and writes it to a file.