JPA, A different object with the same identifier value was already associated with the session

Entity 쪽 컬럼에 값이 유일해질 수 있도록 컬럼에 @Id 추가

The JPA Id does not always have to match the database table primary key constraint, nor is a primary key or a unique constraint required.

https://en.wikibooks.org/wiki/Java_Persistence/Identity_and_Sequencing#No_Primary_Key