import os

with open('db.txt','r',encoding='utf-8') as read_f,\

        open('.db.txt.swap','w',encoding='utf-8') as write_f:

    s=set()

    for line in read_f:

        if line not in s:

            s.add(line)

            write_f.write(line)

os.remove('db.txt')

os.rename('.db.txt.swap','db.txt')