JPA Primary Key From Two Foreign Keys

I am new with java, eclipse, jpa(eclipselink), postgresql, and trying to make a web application. I have two tables:

bids: id, quantity, price

trades: bidid, askid, quantity, price

bidid and askid columns are foreign keys from bids table (id), and they are the primary key for the trades table.

I created the Entities from the Tables (Bid and Trade class) with eclipse and it generated a TradePK class for the primary key.

Trade.java:

@Entity
@Table(name="trades")
public class Trade implements Serializable {
@EmbeddedId
private TradePK id;

[Code] ....

I understand that this is necessary because the primary key is from two column, but as soon I want to persist a Trade back to the database Eclipselink call the column names twice:

INSERT INTO trades (price, quantity, datetime, BIDID, ASKID, bidid, askid) VALUES (?, ?, ?, ?, ?, ?, ?)

So postgres give me back an exception: ERROR: column "bidid" specified more than once

And I don't know why. According to the JPA docs I didn't found any mistake:

Introduction to EclipseLink JPA (ELUG) - Eclipsepedia

Or I just can't see it... or is there a better doc which explains it better?