INSERT INTO user (id, first_name, last_name, email) VALUES (CAST($id AS INT), :"First name", :"Last name", :Email)
ON CONFLICT (id) DO -- this syntax is PostgreSQL-specific. In SQLite, use ON CONFLICT IGNORE.
UPDATE SET first_name = excluded.first_name, last_name = excluded.last_name, email = excluded.email
RETURNING
    'redirect' AS component,
    'edit_user.sql?id=' || id AS link;