current_users=['Niuniu','Niumei','GURR','LOLO'] new_users=['GurR','Niu Ke Le','LoLo','Tuo Rui Chi'] current_users_lower = {user.lower() for user in current_users} for new_user in new_users: if new_user.lower() in current_users_lower: print(f"The user name {new_user} has already been registered!...