python 文本文件中去重复

res_list = []
f = open("ls.txt",'r')
dup=[]
index=0
dul=open("ls1.txt",'w')
for line in f.readlines():
    index=index+1
    if line in res_list:
        print('in list')
    else:
        line.replace("\n"," ")
        dul.write(line)
        res_list.append(line)