MySQL: How to insert a record for each result in a SQL query?

As simple as this :

INSERT INTO config (id, customer_id, domain) 
SELECT DISTINCT id, customer_id, domain FROM config;

If you want “www.example.com” as the domain, you can do :

INSERT INTO config (id, customer_id, domain) 
SELECT DISTINCT id, customer_id, 'www.example.com' FROM config;