how to have postgres ignore inserts with a duplicate key but keep going

If you're using Postgres 9.5 or newer (which I assume you are, since it was released back in January 2016), there's a very useful ON CONFLICT cluase you can use:

INSERT INTO mytable (id, col1, col2)
VALUES (123, 'some_value', 'some_other_value')
ON CONFLICT (id) DO NOTHING