current_users = ['username_1', 'username_2', 'username_3', 'username_4',
                 'username_5']
new_user = ['new_user_1', 'new_user_2', 'new_user_3', 'username_1', 'Username_2']

for username in new_user:
    if username in current_users:
        print("Please enter a new username.")
    elif username not in current_users:
        print("This username is available.")

username.lower() == username.upper()

new_user.lower() == new_user.upper()

lower_case_users = [user.lower() for user in current_users]
if new_user.lower() in lower_case_users:
    # Name clash