txt file duplicate line remover python

1lines_seen = set() # holds lines already seen
2
3with open("file.txt", "r+") as f:
4    d = f.readlines()
5    f.seek(0)
6    for i in d:
7        if i not in lines_seen:
8            f.write(i)
9            lines_seen.add(i)
10    f.truncate()