MySQL:insertar donde no existe

Usa insert . . . select :

INSERT INTO USER (name, email)
    SELECT 'John', '[email protected]'
    WHERE NOT EXISTS
        (SELECT id FROM USER WHERE email = '[email protected]');

Escribiría esto como:

INSERT INTO USER (name, email)
    SELECT name, email
    FROM (SELECT 'John' as name, '[email protected]' as email) t
    WHERE NOT EXISTS (SELECT 1 FROM USER u WHERE u.email = t.email);

Pero probablemente un mejor enfoque sea poner un índice único para que la base de datos proteja los datos:

create unique index idx_users_email on user(email);