PostgreSQL needs to have its Primary Key reset after a table is duplicated - otherwise it runs into the PK ID taken issue. Here's how to reset it to the next highest PK..

pk_reset.sql

SELECT setval('TABLENAME_id_seq', (SELECT MAX(id) FROM "TABLENAME"));

-- Literally replace TABLENAME with your table name:
-- SELECT setval('foo_id_seq', (SELECT MAX(id) FROM "foo"));