You're on the right track with setting up a one-to-many relationship in PostgreSQL using Spring Data JDBC. The issue you're facing is common when using LEFT JOIN, It duplicates the parent row for each child, which isn't ideal since you're using a Set<ChildEntity>.
Since Spring Data JDBC automatically maps child entities when fetching a parent, you don’t need to manually join tables. Instead of using LEFT JOIN, just fetch the parent entity directly or fetch separately the child records separately.
SELECT * FROM parent_table WHERE position_id = ?; or SELECT * FROM child_table WHERE parent_table_id = ?;