Generate a lot of random strings

I have a method that will generate 50000 random strings, save them all to a file, and then run through the file a delete all duplicates. Out of those 50,000 random strings, after using set() to generate unique ones, on average 63 of them are left.

Function to generate the strings:

def random_strings(size=8, chars=string.ascii_uppercase + string.digits + string.ascii_lowercase):
    return ''.join(random.choice(chars) for _ in xrange(size))

Delete duplicates:

    with open("dicts/temp_dict.txt", "a+") as data:
        created = 0
        while created != 50000:
            string = random_strings()
            data.write(string + "\n")
            created += 1
            sys.stdout.write("\rCreating password: {} out of 50000".format(created))
            sys.stdout.flush()

        print "\nRemoving duplicates.."
        with open("dicts\\rainbow-dict.txt", "a+") as rewrite:
            rewrite.writelines(set(data))

Example of before and after: http://ift.tt/2epUkuV

How can I generate completely random unique strings without duplicates?