代码如下：
current_users=['Niuniu','Niumei','GURR','LOLO']
new_users=['GurR','Niu Ke Le','LoLo','Tuo Rui Chi']
for i in new_users:
    for j in current_users:
        if i.lower()==j.lower():
            print(f'The user name {i} has already been registered! Please change it and try again!')
            break
    else:
        print(f'Congratulations, the user name {i} is available!')
总结：
主要考察字符串的大小写转换及其格式化。