current_users = ['Niuniu', 'Niumei', 'GURR', 'LOLO']
current_users_lower = [i.lower() for i in current_users]
new_users = ['GurR', 'Niu Ke Le', 'LoLo', 'Tuo Rui Chi']
for user in new_users:
    if user.lower() in current_users_lower:
        print(f'The user name {user} has already been registered! Please change it and try again!')
    else:
        print(f'Congratulations, the user name {user} is available!')